Goin standalone
 

I'm highly considering buying a standalone computer for my S5 N/A. But I have a few questions. First off I don't plan on doing anything to my motor. Any upgrade I do now will be awaiting a TII swap with the turbo motor being last. Now I have a TII drivetrain already (TII rear end being put in shortly) with an ACT prolite flywheel and stock TII clutch (with maybe 5k miles of easy driving on it.) So soon I'll be buying a standalone system and tuning it. That way if I blow up my N/A motor I won't be out as much as say a ported S5 TII motor make sense? So using a stock motor and fuel system on the stock ports. What kind of power will I gain with proper tuning and will I need to swap the harness when the TII motor goes in? And what is the recomended stand alone for getting close to 200hp on my N/A and 300hp on ported S5 TII motor with the stock turbo?

10-15whp seems to be pretty common. The harness would pretty much be the same, you'd just wire in a knock sensor (if your ems supports it)

Any EMS will get you your desired HP numbers, as the brand name doesn't have anything to do with it. Its all the tuning.

The difference between engine mangement systems will show itself more in overall driveability, smoothness, hot/cold startup, ease of installation and tuning, and specific features.

Good idea.

The power depends more on the tuning and the condition of the engine, and doesn't have much to do with the EMS. What you will notice immediately is that a standalone EMS allows for much better throttle response than the stock ECU, and most people have also been able to gain better gas mileage.

I don't quite understand the wiring harness question. If you are asking about whether an NA and turbo engine would use the same standalone EMS harness, the answer is ... mostly. :) As long as you stick with the same ignition system (CAS, 60-2 wheel, etc.), and the same number of fuel injectors (4?), then that part of the harness will be just fine for both engines. However, if you want to add things to the turbo engine that were not on the NA engine, such as boost control, then you would need to add the necessary wiring. Also, if your EMS harness uses aftermarket sensors, you would need to install them in the new engine.

Use the EMS that has the best local support. Also, you need to decide which functions and features you like, as each EMS is a little different in this respect, even though they will all run your engine just fine.