Hi!
Why there is different gain in power when using pipes depends most on what exhaust timing the engines have. For instance the OS LA series of engines have a very mild exhaust timing of around 140 degrees (the exhaust opening is held open for 140degrees of a total of 360 degrees crankshaft rotation).
And a MVVS Q-500, 6,5cc engine has a somewhat higher timing at around 165 degrees.
And a real powerfull pylonracing 6,5cc MB engine (or Nelson or JETT) has an exhaust timing around 190-195 degrees.

It's easyest to just take a diamond file and try to raise the exhaust opening some 10th of a millimeter, but be carefull so that the chrome/nicasil layer won't crack.

But as ...if you want a powerful engine ...get one new, one intended for the purpose and don't try to get a sport 6,5-7,5 cc engine ( OS, Webra, TT, Magnum, Super-Tigre, Rossi, or MVVS... etc) run like a pro engine like a MB, Nelson or JETT.