Even if the bore were different it would only be marginal, and make little to no noticeable difference to performance unless you were racing, and if you were, you'd need a few more things to worry about than the exhaust size. However, not sure if 2.5s were Stow n Go, and if not the routing could be different, as could the bracketry. I would have thought that there were now more than enough 2.8s been broken, that finding a correct fitting exhaust would be a safer bet.