Lastly, there's no way a $2-3k ECU upgrade is going to yield 425 BHP. To get that kind of power you are going to have to invest much more. Remember whatever addtional HP you that you believe is lurking @ the rear wheels is also @ the M's rear wheels.
I am starting to really feel like I am pushing the 335 on people but if you think that around 420Hp from a tune on a 335 is not possible then you havent visited the 335 FI forum on E90post.com. There are already about half a dozen dynos from 335's that are pushing 330rwhp with only $1300 invested. You do the math but that is getting darn close to 400hp with only a tune.
I am not trying to push the 335 on anybody but I think the realities of its tuneability need to be recognized. Keep in mind that Vishnu is only the first of many companies to mod the 335. I can only imagine what a company like Dinan could do.