Port job will make the most effect for you. I would not bore for power reasons. Stock crank trued/welded best bang for buck. You will be able to hang just fine provided your setup is dialed in. Drag race is another story.. You may consider shift star mod while it's apart.
Sounds like you have an air leak. You MUST do a leak down test if you wanna fix it. Could be seals or reed cages leaking. Do a search for leak down test. Mine used to do that before I finally took the forums advice. I repeat, leak down test.
Hopefully someone has 2x of numbers 12&13 they could send my way. They want almost $20 online. I was hoping someone had extras lying around. I already have new seals and bearings. Anyone?
Also selling grey Maier plastics 300 obo like new
http://www.ebay.com/itm/Yamaha-Banshee-plastics-fenders-Maier-Grey-Silver-/321243913484?pt=Motors_ATV_Parts_Accessories&hash=item4acba11d0c&vxp=mtr
No before pics sadly. I just decided to document the rebuild after staring at the pics on my phone. lol. It was stock blue plastics and grey frame. Fairly stock other than pipes and some porting.