I can't believe they are still selling that relocation harness. I thought it was replaced by the Gmos-04 years ago. I remember when the bypasses first came out and I thought it was the dumbest thing I've ever heard of to have to mount your factory radio somewhere in your trunk.
And as far as a better amp. There would be little improvement unless the speakers are replaced as well. I know it doesn't have the most power but I believe the amp is a signal processor/x-over as well and is matched to the speakers. If you want to keep it simple and are happy with the factory sound, I wouldn't mess with it.