1. How much of a gain would i get from this?
You will lose boost, which can cause a loss of bottom end power
2. Is this under $1000 total?
I would certainly hope that you are not spending anywhere near $1000 for a downpipe and 2 mufflers...
3. Will this sound good?
No. Its a 3800... The only good sounding exhaust retains stock mufflers.