Believe? What are you? A Republican? There is nothing to believe. It's a proven fact. Technique Tuning and TRM make PROVEN turbo kits Stage 1 makes low to mod 300's stage and beyond make a lot more. Reliable, repeatable/ All by lowering static compression ratio via a thicker head gasket. (not the most ideal way to do it, but it works.)
A VR4? as in a Dodge of Mitsu? Those were absolute pigs compaired to the E36 chassis. 320 Hp in an E36 woule smoke one of those...
Anyway, adding any cam kit to a stock S5x will cost you about $2500 plus labor. (cams, hfm, injectors, tuning, M50 imanifold if you don't have one), and you get about 30-40 rwhp. Being generous that's about $85/hp. A TT stage one costs $6500. makes about 330 hp. (about 110 rwhp gain). That's $60/hp. Either way, you gotta pay to play...
No matter where you go, there you are.