I went Twin Turbo on my 2013 ZL1 M6, it was a massive upgrade over the blower. I did all the work myself and spent a lot of time perfecting the platform. The end results were really really good, 1077 rwhp / 1029 rwtq. (lots of videos here:
)
The sounds, the experience, the THRILL of having 4 digit power was incredible!
Pros:
Insane Power
Beautiful V8 Twin Turbo Sounds
Very unique car
Cons:
Lots of upkeep, tinkering, even with 20 years of experience, it will always need a clamp tightened here, a hose moved or adjusted there.
ZERO return on investment, ZERO, don't do this if you want any money back on your TT system, most people are scared to buy a car that has gone through this transformation. Returning it to stock is really not an option because so much is changed/modified to install the systems
Hope this helps!