To clear up a few things.
Cyber-Armor stacks with other cyber-armor. Only attributes are averaged. Enhancements (which is what cyber-armor is classified as on pg. 343 SR4A) aren't. Attribute enhancements add to the attribute of the limb, which then gets averaged, but armor doesn't have an attribute to add to/get averaged.
In addition, targeting a single limb may cause only that limbs attribute (Body) to be used for the damage resistance test, but it would not limit the armor in any way. Armor is not an attribute, it is an enhancement. It is not averaged or subject to the "use the
attribute for that limb" phrase consequently. The only way to negate armor in SR is to call a shot to negate armor or manage to get enough AP to negate it. Armor is not location specific. SR has no hit location system.
Whether you can target a specific limb to force only it's Body to be used is a GM call (all Called Shots are GM calls). The following phrases are relevant here, "pending the gamemaster’s agreement" and "the gamemaster decides if such a vulnerable spot is accessible". Both are from pg. 161 of SR4A in the called shot section.
Toxin resistance uses the character's body. Nowhere does it state unaugmented body is to be used or that only natural body is used. It's a GM call at best on whether or not the cyber-body would help at all, but by RAW I have yet to find anything stating concretely that you cannot use Cyber-Body for the test. To be perfectly honest, we have no idea how the 2070ish cybernetic replacements would work with the blood system. Claiming to
know one way or the other is just blatant ignorance (unless you're working in the cybernetics field and would like to provide us with some insight on what's coming out next year). The blood could be limited to just the meet body, or it could flow through the arms and legs to simulate a normal system and pass through basic filtering systems (higher body = more systems). The book literally does not say either way. Remember, that drugs affect the character completely as well (not just his meat body).
Players and NPCs trying to get out of a -20ish modifier by using Long Shot may often find themselves subjected to the optional rule for longshots. A -20 modifier is supposed to be something that is hard to accomplish. How often did the terminators get blown away by small arms? That's what someone in full blown replacement with 4 levels of armor in each limb is going to resemble damage resistance wise.
As for Adapsin, you can start cybered, gain adapsin and have a lowered rate on the replacements you pick up. So going from say a tweaked out second hand torso that cost roughly 17k and 1.8 essence to an Alpha Torso after the adapsin treatment would cost the runner around 98k (68 for the new torso and 30k for the adapsin) and get him an essence hole of .75 (1.5 x .7 = 1.05. 1.8- 1.05 = .75).
Replacing good standard limbs for alpha limbs would run about 60k a piece and net an essence hole of .3 each. I used a second hand torso because that's about all a starting character can take and have any avail left for any customization.
You can't start maxed out, but you can easily make an out of the box competent replacement character with initiative boost either through synaptic boosters or some speedballed drugs (Jazz + Cram or Jazz+Cram+Kamikaze).
As for trying to build a tank. Remember, the character still has worn armor that he can mod with the elemental resistances (really irks me that there weren't cyber versions of most of them). But yes, a mage seeing that much "dead" space in his aura would probably mana the shit out of it.
For the sheer "Over-powerdeness"* that can occur, see the
Brick Wall that I used for a very very advanced group's end campaign mission. With Gel Packs added to his trouble armor he was rolling 66 dice to resist before counting in AP.
It's powerful, but there are far more attainable and powerful options to go for (mob mind spell for instance or even the
Suit of Doom).
I want a book, page, chapter and verse cite.
How about you do the same for your claims dude. Pointing out that RAW does not equal your line of thinking is not an attack on you. It's just pointing out how things actually work.
All you're doing is exactly what you've tried to say PAX is doing. Muddying things up without any support and claiming that since it's your viewpoint and logical to you, it must be RAW.
edit: fixed math to not include bio-compatibility