Congrats on the 335! It shouldn't be a surprise how much faster the 335 is, I've driven a 2003ish IS300 and it's slow. So a 335 is going to be a world of a difference, especially torque-wise.
As far as the warranty goes, I'd ask your independent mechanic which companies he's dealt with and had the best experience with. Like you said, you may be better off just saving the ~$2,000 for maintenance and repairs.