There is a whole other world when it comes to type IV performance. These engines have been explored and are being expoited to the point where they are now super reliable and uber powerful. One name that stands out in the T4 arena is Jake Raby of
Aircooled Technology.
A really long thread on a type IV forum that has video and lots of pictures during the building as Wallys turbo type IV comes into it's own. He regularly street drives and races it at considerably higher than you might expect HP levels.
Another type 4 turbo: 2,2 ltr. Dyno results "103x66 (2200cc) T4T, 418 hp @ 19 psi"
Wally shows his successes and failures in that thread.
However, like Dave says this is not going to be inexpensive but the results are fantastic.