These engines produces sometimes up to 250 hp and is a 1.6l, normaly. 2.5 is more than enough. When we're talking 3l, six sylinders, giant turbo and 1000 bhp, then 4" is needed. 4" on a RST is just unpractical.

WRC cars uses 2.5" exhaust. If it's good enough for Petter Solberg, is more than good enough for you.