DE's maybe 300+ odd with basic mods.
Engine rebuilds, new manifolds, different cams etc though may yield more but then that's similar money to charging.
Varies between cars though but my old 2003 GT coupe got 294Bhp@fly with a H.Dev plenum spacer, Stillen induction kit, Berks HFC's and an Invidia Gemini exhaust system. Others have broken the 300 barrier though.
Welcome aboard Ben.