Have people tried different design exhaust manifolds other than janspeed. ?
Matt Humphris has.
There are several manifold variants he offers. His Janspeed manifolds, which are essentially what Maniflow made back in the day, have much bigger ports which pushes peak torque and horsepower up the rev range. He also does a longer primary version to help boost torque. I believe he's done some dyno work on them and may have some info on benefits.
Eden from Gizfab does a few variants also but I don't know if anyone has had one on a serious motor on the dyno yet.
I'm also wondering what AP Race Engines may use as they seem to get good results out of CG13/CGA3 engines.