I don't personally have an SRT6, but have been reading the forum for a few years.
There is no appreciable gain, if you do it, it will be for the sound.
Choices :
1. Magnaflow
2. Borla
3. OBXRacing ( Chinese Borla copy at 1/3 the price )

The Magnaflow is a bit louder and has a bit more drone than the Borla / OBX

See my thread about the OBX for some sound clips.