I over filled my bike @ about a 1/4 of a
quart too much and it was pulling like a mutha. I, of course let some oil out after it cooled down, and it seems to pull a little less strong....Why does anyone know?
a) excess oil will negatively impact performance. Your clutch, crankshaft, etc all has to spin through this oil. Excess oil causes excess drag which, subsequently, causes power loss. That's the whole theory behind knife-edged cranks, oil-splash baffles, etc... they minimize drag on such components.
b) If you're venting your crankcase into the airbox (as your bike does stock), higher oil levels will result in more oil (either misted or splashed) being sucked into the airbox. This causes increased carbon formation on & around the valves, increased carbon formation in the combustion chamber, intake dilution, and increases the chance of detonation.
c) higher than normal oil levels can cause significant clutch drag on some bikes, resulting in rough shifting and increased clutch wear.
there are other reasons not to run higher than normal oil levels, but these are just a few. Surely, too much oil is better than too little, but both are bad. I can see erring on the side of caution if wheelies are your thing and you are afraid of the oil pickup sucking air, but overfilling by a quart or more (even 1/2 quart) is WAY too much.