Sorry, I guess I wasn't clear - the fuel cut defender (FCD) won't prevent boost creep, it will just keep it from being catastrophic. With a free-flowing exhaust, the turbo has less backpressure on it, so the turbine makes more power and thus can compress more intake air to a higher pressure. Normally, you could open the wastegate further to bypass the turbine, but the stock S4 & S5 internal wastegates aren't big enough even when fully open to bypass enough exhaust gas around the turbine, so you get boost creep. This wouldn't be so bad, except that when the boost gets above a pre-set limit (6 psi for S4's, maybe 9-10 psi for S5's IIRC), the stock ECU cuts fuel, which can cause a lean condition and lead to detonation, killing the engine.
All the FCD does is prevent the ECU from actually seeing the correct boost pressure, so it never detects the pressure that would cause fuel-cut. Effectively, the FCD passes through the manifold pressure signal unchanged up to atmospheric pressure, and then starts scaling it once you're boosting. Since the actual fueling rate is based on the AFM, which detects the extra air flow that goes with the additional boost, you get enough fuel delivered by the injectors, BUT the spark correction, which is also based on the boost as well as air flow, can get too aggressive if you push the boost beyond a few PSI over stock. Even so, I'd run 93 octane exclusively for the extra knock protection since the spark will be slightly advanced when boosting.
With that being said... $130 for one?! Holy crap, its a couple of resistors and connectors! I used to have one, let me dig around & see if I can find it for you.
Also, DO NOT use the S5 NA AFM - it will be rich at low to mid loads, right up to the point where the turbo maxes out it's flow capacity and then you go lean, popping the engine! For example (and i'm sure the numbers are wildly inaccurate), say the S5 NA flows 100 cubic ft of air/minute (CFM). The ECU will want to see that whole range from 0 - 100 CFM over it's voltage scale of 0-5V, so the S5 NA AFM will show 0V at 0 CFM, and 5V at 100 CFM, at which point it is max-ed out and can't read any higher. Now, if the S5 Turbo flows 150 CFM, then the S5 Turbo AFM will have 0V at 0 CFM but 5V at 150 CFM. If the S5 turbo ECU is expecting to convert 5V into 150 CFM, but it's hooked up to an S5 NA ECU telling it 5V (which is actually 100 CFM), the ECU will deliver 50% more fuel than is necessary, making the engine run rich. The dangerous part comes when the engine actually flows more air than the NA AFM can measure - at that point it just keeps reporting it's maximum airflow and the ECU doesn't add any more fuel or pull any more timing, and the engine runs lean & goes "POP". This is a pretty simplified explanation, leaving out all the correction curves that convert between CFM and voltage, temperature correction etc, but you get the picture...
There is a bit of a difference between S4 & S5 alternator wiring, but IIRC it amounts to running 1 extra wire. I've swapped the larger output S6 alternator into my S4 'vert, and it just needed a voltage signal wire from the fuse box next to the battery.
One thing to be cautious of with S5's is the electronically controlled OMP - if it works well, then great, but they seem to be a bit of a failure point vs. the S4's mechanical OMP. Then again, when the S4's OMP fails, you don't necessarily see it as an OMP failure, you see it as an engine failure...
