Need some help with track day tire sizes--987

[MikenOH]

've got an 06 Boxster with an extra set of 18" rims and standard PS2's that I use for DE's. These tires have worked well for the purpose of having a tire that you can drive to the event on and use at the track. They've worn well and, while not as sticky as R compounds, can take the heat, handle predictably and have a sufficient amount of dry traction to have fun.

These tires are about done and I have a chance to get a set of Extreme Performance tires for essentially nothing, but can't get a rear tire in my stock size--265/40/18. The closest I can get is a 275/35/18 which is about .6" less in OD vs. stock.

My question is: will this smaller OD tire create problems in handling at track events? I like the idea of a free tire set but not at the expense of creating problems at high speed events. If this were for auto-x, I wouldn't think twice about going with the smaller tire, but..

I plan to keep the 235/40/18 size on the front; the replacement tire has performance similar to the Dunlop Z-1.

Thanks for any help.