Thermal, Mugen, and Apex WS are nice choices IMHO. The Thermal R&D exhaust for the 5th gen is dual tip similar to OEM and is very deep. Personally mine got annoying on the high way but others find it to be fine. I swtiched to the WS since it was said to be the quietest out there and it seems to be still. There are some times where even my WS gets a little loud but that's the nature of the game. Mugen is very nice, it makes good gains, looks good, and sounds nice but it's big bucks for being so well made, imported from Japan, and of course to pay for that Mugen name that you get with it.
I've never heard the Borla and thought it was only an axle back making it a free flowing muffler and not too much more.