How long did you have the bike running 'well' with the new engine mods (compressions, reeds, filter etc) before you changed the exhaust o-ring / gasket?
I'm no expert on these matters, but if its choking at top end, with the exhaust leak now sealed, your pipe itself could be partially blocked. If the combusted gases are not exhausted out of the cylinder properly after detonation, it will run hotter.
_________________
Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30