5 gangers square off against the street sam and they all shoot one initiative after the other.

Ganger #1 Initiative 12 - Suppressive fire (-0 DP first attack)
Ganger #2 Initiative 11 - Wide short burst, and Narrow Short Burst (-3 DP)
Ganger #3 Initiative 10 - Two Single shots (-2 DP)
Ganger #4 Initiative 9 - Wide Long Burst (-6 DP)
Ganger #5 Initiative 8 - single shot
Runner #1 Initiative 5 - has -11 total dicepool modifier when resisting Ganer #5's attack?

so in a High velocity weapon you can fire 2 long bursts in a round and hit up to three people with a wide burst (withing a couple meters).
what is the downside to the initiative master of the group throwing wide bursts at all the enemies (HV, ambidexterity, Akimbo style)?

First: The dice pool modifiers are not cumulative. They apply individually to each attack. The only thing that is cumulative is that you have a -1dp per attack after the first (Defensive modifiers BBB pg 151) until your pool refreshes.
So, versus ganger #5 he has a -6dp.

Each attack you defend against after the first imposes a -1 penalty. Since runner #1 has defended against 6 attacks when ganger #5 shoots at him, he has -5 to defend.

The penalties from wide bursts do not stack. They only apply to that specific attack. The only thing that stacks is the -1 per attack defended against penalty.

There were actually 7 attacks, less the first one = -6DP from Dodging

The super-duper muchy move that no GM should let fly is to split your dice pool umpteen ways. Let's say you have an Agility+Heavy Weapons+Mods= 12 dice with a machine gun. Split that into twelve one die attacks on 1 person. Now they are at a cumulative -12 to dodge. Then with your next simple action you pull a narrow burst to the head ... and let teammates and drones have your way with them

sa = successive attack
Ganger #1 Initiative 12 - Suppressive fire = threshold test
Ganger #2 Initiative 11 - Wide short burst(-1 sa, wide -2) and Narrow Short Burst (-2sa, +2dv)
Ganger #3 Initiative 10 - Two Single shots (-3 sa and -4 sa)
Ganger #4 Initiative 9 - Wide Long Burst (-5sa, -5wide)
Ganger #5 Initiative 8 - single shot (-6sa)
Runner #1 Initiative 5 - is probably screwed at ganger 4 lol with a -10 mod to dodge.

You cant split dice up like that Weaver. You can call your shot and withold dice to jack up damage or split the pool when your dual wielding but nowhere does it say you can make each individual bullet its own attack. You can do that with spells i think heh. .....

If you're making each bullet it's own attack, you're firing in Semi-Auto mode, meaning you get 2 shots. That's it. Nice try though.

EDIT: Of course, if he has a cybertorso with 11 pistols mounted and DNI'd to be usable individually, I suppose he *could* split it up into 12 shots (11 from those guns, plus the one in his hands). That would work, but that's ultra-munchkin, and at that point is probably using autosofts to get the guns to fire together.... Bah, that's too complicated.

Yea, at this point he'd better be a big ork or troll, heavily armored and cybered (platelet factories, dermal, etc)

To be fair, what sort of Street Sam has an initiative score of 5?

A dead one...

A good samurai will have a reaction of 8+ for defense. With multiple attackers, the samurai will want to spend at least one IP on full defense nevertheless.

Each consecutive attack reduces his pool, and each attack should have its own burst modifier. First bursts wide to increase the likelihood of a hit, later bursts against the reduced pool narrow to increase damage. Works fine. Drones can play this game surprisingly well, their attack DPs are not that low.

there are several things wrong with that. first, as others have pointed out, you can break full-auto fire into seperate bursts with one action only if you are actually firing at multiple targets. second, you can't break it up into one-round bursts; you have the option of breaking it into either two or three bursts. third, in order to do that, you have to fire a full burst, which is a complex action--by the time you get your next simple action, the DP modifier for defending against so many attacks would have faded anyway.

More likely, the runner sammie at Init 20 took his action to kill one of the gangers, and went from being outnumbered six-to-one to being outnumbered five-to-one. Being outnumbered sucks in a wide variety of situations including some real life gunfights. The gangers might also have someone going two-gun to multiply the distraction attacks. With enough attacks, sooner or later the defender will glitch.

What sort of Street Sam gets an initiative of 20 and is still in a position to be shot at after his first IP?

I refer the honorable gentleman to my previous answer....