A bigger exhaust port means more top end power. Unfortunately when it gets really big, piston stability is reduced and you can get scoring on the piston where it catches on the edge of the top of the exhaust port. The MES design is just a really big exhaust port with a couple bridges in the corners to reduce the chance of piston damage.

On another note, I used to run a high carbon fuel that could actually plug those ports up with carbon build-up after about a gallon. When that happened, power dropped noticeably. Regular tear downs for cleaning kept it running great.