So, clearly an extra IP isn't as good as a whole extra person. It doesn't give you a second body track, and it makes you act 'slower' in that all the guys in IP 1 get to shoot and then you get your second shot.

However, is a guy with 4 IPs better than 2 dudes with IP each? Probably, he has twice the firepower but half the hitpoints, and will go first in IP 1.

So how much of a force multiplier is each IP do you reckon?

I would put each IP at around a +75% combat effectiveness to a character vs not having extra IP. You're right in saying that it's not a whole extra person but the stuff that gives you extra IP also strongly tends to making it harder to get a good hit on the character so it also increases their survivability. My ganger/sammy has 3 IP and he's easily 250% of what he would be with only one pass.

I've had a great many combats where the phys ad with "I cast Shitkick I-beam" got in his Counterstrike/Riposte on his IP to blow out a single enemy then my ganger finished his IPs to take down 4-6 of the rest of the worse guys.

But then, that's what he gets for making a gimp-ass phys ad that can blow out a Streetmaster in one hit...

The survivability is greatly improved if you have an IP or two to spare for full defense.

Fuchs is right. Going full defense until everyone else is out of IP's is a valid, and effective, tactic. It also gives you the opportunity to gauge which opponents are the most dangerous, and plan accordingly.

Against single IP foes, I'd use the first IP to kill one or two enemies, then interrupt full defense to evade their attacks, then use the third IP to kill more. Repeat in next phase.

Yes, but neither you nor your PC should know how many IPs the NPCs have.

We don't - for one round. After that, you know, as player and as character, how fast the enemy is.

That would depend actually on what the NPC does during that one round. If the NPC does something that the PC can perceive as an action, then yes, the player would know that the NPC has at least so many initiative passes. If both PC and NPC are in Full Defense for all of their IPs, then it might be likely that neither side knows how many IPs the other side really has.

And if they're hiding about in full defense for the whole round, then they obviously aren't trying to kill the PC. So the PC will probably shoot the other ones first, especially considering he can always interrupt to get full defense.