Make my 3.5" exhaust on the HR31 is 90-91dB, I can assure you it's quieter than most 3" systems i hear around here.
Whenever i go on a cruise I'm totally stealth thats how quiet it is.
4" isn't much worse either provided you get GOOD mufflers and a big flowing CAT.
Owning a GTR isnt cheap, nor is chating 350-370rwkw... If you need a 3.5" or 4" exhaust, then pay for it
3.5" should do you for your setup either way.