I waited until I needed a new clutch. I disagree with MES. While it's not the seat of the pants feeling that I imagine a 3:54 rear gear offers (wouldn't know, I still have my 3:07), the car definitely runs up through the rev range faster and feels peppier in 2nd, 3rd and 4th. Is it huge ? No, if you are in there any way, sure.
Caveat: I definitely have to launch at higher revs if I want to replicate the surge out of the hole that I sued to feel, but on a road course, the car definitetly feels friskier.