The title on this post is a bit of a misnomer. These are more like "common somewhat obvious especially when you search" questions but that doesn't make for a good title.
Stupid question #1: Can I run a blow off valve?
Yes, you can. Just make sure it's a dual piston BOV so that it doesn't leak boost on idle. If you bought one that isn't the aforementioned design, you'll know it, since your car will struggle to idle and stall.
Yes, blow that $250 SSQV that EVERYBODY and their mother runs because it makes a "pshhht" and a whistle out your ass and buy a Forge 004 for around half the price. It works awesome and makes the straight up "pshhh" that we all love, no annoying whistle necessary. Don't go eBay.
Which one performs best?
As long as they all hold boost and respond fairly quick there's no real performance difference. Some people make a big deal out of response time, but for an average driver I don't think it's a concern.
Stupid question #2: Can I swap the engine from a 225 into my 180?
Swapping the 225's engine into a 180 would be idiotic. The only significant differences between the two engines are the turbo setups and fueling which can be changed out easily. ATC code engines use identical internals to 225 engines.
What all other engine swaps can I do?
The only one really worth doing would be a 3.2 VR6 swap from the S-Line TT. For this, I'd speculate the major parts you'd need would be an R32 bell housing to mount it to the 02M (quattro) transmission along with all the electronic goodies to make it work. In reality, any engine put in a MkIV VW could be used; the 1.9TDI, the 2.slow, the 2.8VR6, but none are entirely simple or cheap endeavours.
What about the 2.7T engine from the S4? I saw it done.
That was an incredible undertaking. That engine barely fits under the hood not to mention the entire driveline on the car had to be swapped out. All in all it likely cost around twice what your car is worth to begin with. Just use a VR6 and call it a day.
Stupid question #3: I hate Haldex quattro! Can I make it rear biased or make my TT RWD?
No, it is mechanically impossible for Haldex to transfer more than 50% of power to the rear. You can't make your car RWD without moving the engine to the back, kinda like the Bimoto.
Can I still make it better somehow?
If you're still pretending like you're going to be drifting around corners on the street in your TT, you can see my post about Haldex in the archives.
Stupid question #4: How much HP does an intake get me?
Not much, I'm talking less at the crank than you could count on your fingers if you were a Sesame Street character. Your car will sound faster though which ought to help you perceive it as a worthwhile investment.
WTF? But on this mad tyte Soobawoo STI an intake made it put down lotsa more WHP on a dyno!
In case you haven't noticed, your car isn't a Soobawoo. Also in case you haven't noticed, the centercaps on your wheels are about as large as your turbo. It's a bit hard to restrict airflow on a turbo that small the same way as it's hard to suffocate an ant by making it breathe through a straw.
Any more stupid questions? Feel free to add them.