Relax. Its all good.
Overfilling will result in the excess being expelled and yes, it does end up in the airbox. Over time there is the risk of fouling the TB's so the butterflies don't close fully. This can be problematic as if the butterflies aren't fully closed after they have been balanced and the TPS has been re-set the ECU will be delivering an inadequate amount of fuel for the amount of air being passed. In turn the stepper will try and govern the engine to target idle by shutting off feed but this can cause an over-correction so the engine will try and stall, the ECU then panics and whacks open the stepper and the revs rise again leading to a high and surging idle.
The thing is that this is something that is cumulative, it won't present as a problem unless you are one of those anally retentive weirdos who at the end of every ride puts their bike up on a stand and adds oil with a pipette until it is *Exactly* at the full mark, (Which is in fact an 'Over full' mark!
). Over a period of some tens of thousands of Km it will gradually present as a problem. The factory recognises this and nowadays recommends that the TB's be removed and cleaned every 10,000Km service. I personally think this is absurd overkill. When I swapped my TB's at close to 70,000kms they were fairly grubby and certainly with the new ones on the bike was noticeably happier and crisper but it had never had any real *Issues* prior to this. I would think that removing them and giving them a damned good seeing to every 50,000 would probably be a good idea but really, worrying yourself silly about the fact that your bike may of been slightly over-filled is un-necessary.
Keep the oil between half way and the 'Add' mark, drain the airbox tube every 10,000km service, ride the snot out of it and stop worrying. Its a GRiSO! You can't kill it with a shitty stick!