Compression is different. Rods and pistons are stronger in the SRT6. The SRT6 has an intercooler and it's intake manifold is different to accommodate the Super Charger as stated above. I'm pretty sure it runs a different grind on the camshafts too.

Many other things on the SRT6 are different as well. Heavier brakes, stiffer sway bars and springs/struts.

HDDP would probably point out anything we missed.