I kinda like the 3.2L S/C swap into the crossfire, be cheaper. And you would have a 330hp 6-speed XF. The all you would have to do is some ecu changes and a pulley, and you could achieve your 400hp goal. That car would be good for low-mid 12's. Now thats what I am talking about. Considering a stock slk32 can hit high 12's stock. So knock off 200lbs, add the advantages of stick, and about 360whp. Be one hell of a car.

Or there is always the alternative of buying a slk55 :-D or a C6 if you want the performance. Though I love seeing modded cars.