Loss Of Power
 

I have a 2008 Audi A4 2.0T and as of late I am getting little to no turbo response. My horsepower has dropped significantly and according to the temp gauge, the engine is running quite cold. However, I am getting the "coolant system performance" engine code and as a result of this, I tried swapping the coolant temp sensor with a new one to potentially fix the problem to no avail. My next best guess would be the thermostat itself but I really am just taking a shot in the dark. The two things might not even be related so if anyone could shed some light that would be a huge help! Thanks!

Sounds like you're on the right track. Fail-safe thermostats can only fail on the open position, thus keeping your engine cool and preventing an overheat situation. I would definitely change the thermostat as it looks to have failed in the open position.

Still having the issue
 

It's really hard to know if you're not throwing any codes. Any deviation from the expected boost should throw a code, so it may not even bee a boost problem. A couple of things I would suggest that may be stabs in the dark:

1. Check the ram air duct leading to your intake box. There's a pre-filter there that prevents your air box from filling up with debris, but it can get clogged up. I had a power loss situation and discovered that the pre-filter was completely plugged with Maple tree seeds - good 'ol Canadian Maple trees ;P

2. Have someone check your vacuum system. A leak in one of your vacuum lines can cause erratic throttle response behaviours

3. Perhaps find someone with VCDS or similar to do a more in-depth scan in case there's a situation recorded but not throwing a code. There are some tolerances in the system that won't throw codes, but they'll be logged. For instance, my car would misfire at least once or twice per cycle for each cylinder, but I never saw a code. I discovered it in the logs.

Good luck!