I don't think you're crazy :) or even very far off base ;)
Though I'm a complete bike mechanic newb, Going over the manuals and other materials about the 220 (I have an 04 also) I see it kind of like this...
Normally, the clutch assembly is partially submerged in lubricant (I guess that's why they call it a "wet" clutch) and for something that spins at the high RPM that the clutch assembly must spin, even small variations in lubricant level would have a measurable effect on the force that might resist that motion (oil level)
The only analogy I can think of is a pretty silly one, but remember as a kid in the tub when you splashed by spinning your hands in front of you in a paddle wheel motion? If you turned slowly it didn't matter how deep your hands were in the water, it took the same energy, but if you tried to move your hands as quickly as you could, it got exponentially more difficult the deeper in the water your hands were...
In the clutch case of the 220 I think there is something similar going on with the added slant that I honestly believe that the clutch and the recommended oil level were both arrived at in order to have the clutch spinning freely almost completely OUT of the oil when running at speed. The lubricant would be "slung" all around in the housing and provide good protection and lubrication by just being everywhere in there at once :) Only when returning to idling speed would the bottom of the clutch then be dunked in the oil...
I think that if you overfill too much, even at speed, the clutch can't sling itself free from the oil level and has to "paddle" its way thru the extra resistance of the oil. If a crank was involved, as in a 4stroke bike or car, this would be called "frothing" and involves SIGNIFICANT power reduction, but I think that to a smaller but still measureable degree it happens with the wet clutch, and using heavier oil as in your case, the effect is amplified...
Just rambling now :) but In my case, I use the whole quart, but then again, I'm a newb who probably only uses 20% of the bikes potential, and I've been filling it with a quart almost from day one, so I have little to base a comparison on... Plus I added an aftermarket pipe, silencer, RAD valve, so this perhaps more than counteracts the effect...
But bottom line, I think your observations are valid...
Have a good one!
Feanor