I would leave the main were it is and move the needle again.If it's getting better sounds like your on track just make sure you check your plugs.On my banshees every time I've had a mid bog they have always been to lean not to rich.
Wow,You should be killing 450's.My next thought is porting is the "gor porting" for bottom and mid power?If it is there's your problem.My o4 shee kills 450's and there is no internal engine work at all.
You don't want to go to .80 over at this point.If your cylinders are on the stock bore "64.00mm" have it bored to 64.50mm.If you go to 80 over the next rebuild you will be sleeving the cylinders or buying new ones.
Defenatly get different cylinders,I believe it's like $240.00 to sleeve cylinders and then you still got to buy pistons and fix the threads in the cylinders.My .02 cents.
A couple of diffrences are the after market guide can be adjusted for chain angle(IE)longer or shorter swingarms and the bottom peice where the chain rides on the guide can be replaced.Your choice.