Does a difference in karma between characters unbalance the game? if yes how do you determine how much is acceptable? I know it applies differently between character types with some being more karma dependent. Also, as a GM do you impose any limits between how much the top and bottom characters?

I would also ask the same question but in regards to Nuyen?

We have had characters with a 60 karma difference in the same group without problem. Cash wise? Would be a guess but I would say around 75k.

In our current campaign, we are embroiled in the Artifacts series. Currently on "Midnight", I think.

We have the following SR4A Characters:

Jet - Face Jack of all Trades, ~410 Karma, Luxury Lifestyle that runs a Minor Triad loyal to the Red Dragons, Millions of Nuyen invested into the Triad.
Jenks - Face Occult Investigator Mystic Adept, 312 Karma, High Lifestyle, Hundreds of Thousands of Nuyen Saved
AWOL - Latent Mage Street Sam, ~320 Karma or so... Lieutenant to Jet in the Triad, and Mercenary Leader
Rudi - Combat Mage, 350 Karma; Luxury Lifestyle, and High Stakes Investor.
Tusk - Hacker Cyber terrorist, 50 Karma, Broke and looking for money, but doing okay.
Ben - Healing Mage, 5 Karma or so; Hungry for Funds
and a Bounty Hunter Street Sam, 5 Karma or so... Hungry for Funds.

Been working well so far. Ben and the Bounty Hunter are new Players just getting back into the system.

We tend to run the gamut, with experienced characters alongside new characters. Especially once a character dies and a team member is sought.

Shadowrun can tolerate power differences pretty well. The trick is to have the lower powered character bring something to the table that no one else does better. Adding in a chargen level rigger when there's no rigger, or a competent medic/doc, or even just an extra guy with a shotgun and a few unique contacts can really add to a veteran group.

Also a backup hacker or mage with some utility spells is always welcome.

The current group I'm in has one simple rule in regards to karma gained:
Everyone is at the same power level and balance level.
GM does not want to be fucking with people on different power tiers.

Not sure how it goes for cash, but I'd assume the extra karma could be dumped into cash a la karmagen for cashbound types.

A higher karma character will potentially have a combination of "Higher Scores" and "More Scores". The nature of karma makes it less frequent to upgrade as you go higher up the chain, and if the GM is using time gameplay related to skill/etc improvement helps keep the spread of increases down too. Even if you had a block of +400 karma, if that came AFTER chargen then the rate at which you go from "Initial Player char" to "Prime Runner" can take quite a bit of in-game time.

So its not just an issue of how much karma do they have, but how much in-game time has passed for the different characters. For Tym's above example, err for example, the higher karma guy not only has better resources, but in game clout due to gameplay, likely has a huge list of contacts/etc compared to the lowbies on that list. The #1 guy might not have super-stats compared to the new char, but has more depth of options potentially.

Honestly, it's the sort of thing that totally comes down to the players and the GM. What works for some groups (like mixing 5 karma and 400+ karma dudes at the same table) and might be awesome for them would totally not fly with other groups (who might be, for lack of a better term, more competitive with each other, I guess?) who are more worried about fairness and equality and junk.

Some tables are really worried about player balance and everyone getting their share of the spotlight, other groups might be more worried with telling a cool story and if someone falls into a sidekick role (or whatever you want to call it), so be it...some groups are in between...life's beautiful and complicated that way. It's the sort of thing that only a given game table can answer, I think.

Our group keeps a Karma/Cash log similar to the one from Missions. New characters get the average value of everyone.

Power is also relative, I have found...

Take the Occult Investigator Face MysAd above at 312 Karma. Magic is split 3/2.
He has 4 Initiate Grades (Flexible Signature, Masking, Extended Masking, Centering - for 44 Karma)
He has 48 Additional Spells over Chargen (Total of 58)...
He has only put 18 Karma into Skills over Chargen.
He has 278 Combined Loyalty/Connection points worth of contacts (more even than the 400+ Karma Character does)

He is socially diverse (10dp for most Social Skills), and quite influential as a Face, but he would be completely squished by the combat Mage (or any of the Street Sams, for that matter), who throws more than 2x the number of Dice for Combat Spells and Pistol Attacks (18dp) to my Combat Spells/Pistol attacks (8dp), is cyber enhanced (3 points of Essence Lost to Cyber/Bio Enhancements), and has a Magic of 6, with 6 Grades of Initiation and a Force 5 Invoked Ally Spirit.

Where my Mage shines is in the sheer diversity that my spell selection gives me. I have not really increased my power level (No Magic Increases and very few increases in skill), but My diversity has grown 5 fold over the combat Mage. I can't kill anything with my magic (Geas Restricted), and I only have 2 Combat Spells (A concussion blast spell and Spirit Bolt), so when it comes to Killing, I am not the one to turn to, the Combat MAge, or pretty much any other member of our team, is the one for that. Hell, even the new Medic Mage has 50% more dice in his field for Magic than I do (My Specialty is Magical Protection and Manipulation Spells, though I have a good number of non-manipulation magic to fall back on).

Those two 5 Karma characters are a great asset in a lot of ways, even if they are starting characters. They still outshine some of the characters in their specialties, and though the Bounty Hunter may not be quite equal to the Cyber Street Sam, she is quite good at what she does.

Every one is useful, regardless of the build...

Now, that being said, The average DP in the group is likely to be around 12 Dice, with some a bit higher (the Combat Mage with DP18, Cyber Sam with DP15 in any Weapons). And that is why it works for us. Had we been playing overly optimized characters with 20+ DP's, the new characters would have had a hard time contributing, becasue they would have had to equally optimize to be useful, and that hurts you in a build when the other characters are both broad (LOTS of Skills) and deep (Decent DP's for most of those skills).

All in all, its been fun, even if we tend to get the snot kicked out of us in the current campaign arc. We still manage to bring it through in the end.

In AD&D we used to give new characters 60% of the groups average XP. Or similar games like Earthdawn.

In games like Shadowrun or Star Wars D6, they started out new.