Yeah those headers are pretty efficient for OEM, but hey what do you expect from a benz.

The only improvement would be to the entire exhaust system after the headers. And opening up your intake. Not sure if SRT6 uses an intercooler, but since its Mercedes It probably does, so a larger more efficient intercooler might help.

I don't really see any need to mess with the SRT6, it's pretty much done except maybe computer tuning. And some weight loss, I would just leave it alone.