Check this one out Gomerkfx
http://www.ebay.com/itm/yamaha-banshee-With-Gsxr-750-Engine-/221204895103?pt=Motors_ATV_Parts_Accessories&hash=item3380d6d57f&vxp=mtr
Yeah, but everybody has had one. lol The selling point was that the boost bottle would store pressure from the reeds snapping shut, then when the reeds opened again, Bam, a boost of air/gas pushed into the cylinder. They sold a ton of these things
The comp tester needs to thread into cylinder the same amount as your plug would. If the tester goes in deeper than plug would or less than plug would, you will get an inaccurate reading. Your problem could be one of many different things. Start with the simple stuff first. You don't need any gaskets to do a leakdown test or compression test. No gasket to sync carbs or clean carbs.
Are carbs clean? check choke cross over tube. It needs to be on even if bike is warmed up. Sync carbs. Check both cylinders compression and compare. If someone you don't know said they rebuilt the engine, then you need to check these things along with jetting and a leakdown so you will know what you have