I was just telling Barry today, while we were cursing at something on the E89 Z4, "I should have just bought a Vette." My dad's a vette guy. I think the new stuff is cool, but I'm just not a vette guy. Plus I enjoy the thrill of building the car. And at 3,100 lbs with as low a CoG as the Z4 has, when I'm done I think this car could be a monster. Maybe. Just maybe.
The N54 platform is a BMW modder's dream - just look at all the aftermarket development going on, and the kind of talent floating around sharing information. BMW guys have always had performance car capability, but not near supercar capabilities.
One more observation - you say "keep it simple" but you want to run two fueling systems (gas and meth). I think with the new flexfuel system that just came, you can seriously keep it simple. Just upgrade your lines (and LPFP if you're an E82/E90), and run whatever fuel you can get. I'm not crapping on meth, but adding meth and "simple" don't go together in my book.
Filippo