i just checked both 2IS and 3IS, it appears the SL models share the same tyre specs @ 255/40/18, but the 3IS FSport does run 255/35/18.
not sure why this is the case.
given OP car is a 2IS, 255/40/18 would be the ideal size to run, as per factory specs. assuming wheel sizes are the same.
as strange as the specs are for the rears, they are factory sizes.