the car can easily make 400whp, it's just a matter of how much $ you wanna spend, and how much you wanna compromise the reliability. the more you put into aftermarket parts, it's more likely your car won't last as long. but if money, reliability, and warranty aren't an issue for you, here are some basic ideas:
- uppipe, downpipe, full turboback exhaust
- agressive ecu tune on 93 octane
- replacing turbo's with a bigger one (one bigger turbo has the potential for more power and better numbers to flex your e-peen, but won't necessarily be faster 0-100 or in the 1/4, because with a bigger turbo, you will lose the great torque at early rpm that the two small turbo's provide for, and turbo "lag" will definetely be 10x more present than it is compared to stock.)
- larger intercooler
- lightweight flywheel
those are some basic not too expensive mods that will provide you close to your goal. any further than that and you're looking at engine rebuild and forging of internals. pricey shit. the problem with getting big numbers with bmw's is that their engines are already so sick from the factory. i mean, a chevrolet Z06 goes 0-60 in 3.9 seconds. a 335i with vishnu's stage 1 on 91 octane can reportedly go 0-60 in 4.2. with a few other mods it, some nice tires, and some weight reduction (take out spare tire etc.), the 335i could probably hit the 4 second barrier, which is EXTREMELY impressive. however, any further than that and you're definetely compromising reliability.
but seriously, an agressively tuned 335i should be able to take a 500hp Ford Shelby GT500. not to shabby since musclecars are supposed to be the fastest in a straight line.
that's about all i can think of atm, hope it helps :_)