I've actually jumped up to 18's on my GT40.
Rubber choices available on 17 & 18" are amazing, plus the performance.
It all depends on what looks you are after I suppose, I originally did not like the look of 16" wheels when they first appeared on Cobra's, then got used to them and then didn't like the jump to 17" and then 18"...but if you pick the right wheels, they can look seriously good.
Seems the shape of GT40 works better than Cobra with larger wheel diameters and more modern wheel designs. Or maybe I have got used to seeing all the GT40's up here with 17 & 18" wheels.
I'm actually going to go the whole hog and fit modern 2 or 3 piece fully polished wheels. The cost is killing me, looks like I might not get any change out of $5k for wheels alone as I would like to get 13" rears for 335/30ZR18 tyres I have.