But an interesting dead horse. The ability to fit really big shoes with a little effort in a car that starts 50-50 with a relatively low CG puts the S2k in an interesting place. Many other cars just don't have the rubber available in the right sizes...or the fender issues are bigger. And the S2k starts with VTEC which uses roller lifters that allow aggressive cam profile the flat tappet cars (2JZ, Nissans) can't touch.

 

Yep id say 9 times out of 10. The curve ball is the 275 is a taller tire effecting the gearing in reducing trq multiplication, and could in some cases not be as effective, but in other instances could be more beneficial if the added gearing helps mph in a gear that would normally run short before the next turn. In a perfect world more tire width/added grip means an increase in cornering speed, which could mean a taller gear/more mph would be beneficial. If you really want the fastest set up, it would all be based on the specific track anyway. If you are running a short course or auto x, you may never get much opportunity to stretch the cars legs enough to see much benefit in a 275 rear vs a 255.

A 315/35/17 has the same overall diameter/gearing as a 275/40/17, so to me it just makes more sense to run the 315, and I do, no matter whats up front (within reason) Suspension tuning balances the traction.

 

I'm quite happy with my choice to go square 255 and on the track haven't experienced any downside. I also noticed the reduction in steering feel as Mugen_is_best says. On the track I don't really care, but on the street I'm driving the car for the fun of it so went with staggered 225/255 using a relatively narrow tire (Firestone 500). There's still plenty of front grip for driving on the street.

But I think we've conflated two issues:
1) The S2000 is faster on the track with more rubber in front so upgrading 225 to 255 delivers real benefit with no downside. I haven't seen anyone disagree with this.
2) Is it because the setup is square that the car performs better, or is it because there's more front grip? There's debate left on this one as others have found 255/275, or even wider, to be a good combination for their setup. I don't think this one can be decided for certain without some form of controlled testing.

 

You can probably get your steering feel back by running the 255 on a 10" wide wheel square, or running a 245 on the 9".