I just fitted my JWT the other day onto a stock GT4.
I have read that it makes a big difference to sound but in all honesty I dont think its a major difference. It goes give a deeper growl and sounds better but its not an amazing difference.
Dont get me wrong, I would recommend one as its a nice difference but a new exhaust is required also to really bring it out.
My friend has an S2000 and it roars when it hits high revs. After the JWT was fitted to the Z its very similar.