I owned an SRT6 about 3 cars ago (SRT6- CSRT4- 09 Corvette- 335xi) and now own a BMW 335i X-drive. Stock it's relatively fast, however it's smoothness makes it feel slower than it its.

A cattless dp, and tune really wake this car up. Now acceleration feels like my old 12.1 second SRT6, in fact I can launch it even harder with the AWD.

Performance-straight-line-wise, I'd say a tuned 335 is really close to a modified SRT6 with the edge going to the 6. However handling, and steering feel are in the BMW's favor hands down. Also much more comfortable and better fit & finish inside and out. Wonderful car, however I'm already plotting to get a Merc C63 AMG now. Those cars are n/a and in the 10's with full bolt-ons!

Here's a pic of my 2011 335i x-drive: