Here is what Renntech offers for the engine in the SLK 320. I don't know if these will be offered for the Crossfire yet, but I e-mailed them, and I'll see what they say. I put the name of the modification, the hp increase, the lb/ft. increase, the total hp and the price. Each package includes everything in the previous package.

ECU Performance Upgrade - 10hp - 12lb/ft - 225hp - $1,595
Carbon Fiber Air Box - 15hp - 14lb/ft - 230hp - $2,495
Sport Muffler - 22hp - 24lb/ft - 237hp - $3,350
3.8L Torque Conversion - 47hp - 65lb/ft - 262hp - $9,985
Camshafts and ported/polished cylinder heads - 76hp - 73ft/lb - 291hp - $14,995
Supercharger and intercooler - 179hp - 181ft/lb - 404hp - $22,995

Obviously all of these are very costly, and I don't know if they're really worth it. Although for the price of an SLK 320 you can have a Crossfire with 262 hp and for the price of an SLK32 AMG you can have a Crossfire with 50 more hp.