Have you looked into putting the stock muffs back on using a reducing 3in>stock join/bit thingy.
I think I've seen a grimspeed unit on carmods previously, tho can't recall exactly.

I could be wrong but I don't think you'd need a retune either.
Might be an option to restore some sanity whilst you weigh up the options.
By all accounts it won't cost you much power either.

Search for 'stealth back exhaust'

Edit - from my limited experience of this issue, size matters. Generally speaking, the larger muffs are the quietest.