could be icing in the intake trumpets (i take it you do still have some fitted). whem the air enters the trumpets, it's speed increases as it is gulped in, this makes it cool even further, and then ice forms, resulting in restricted air flow, over rich mixture (possibly) and loss of power.
mate of mine used to experience similar power loss symptoms on his zzr600 bike a few years ago (the one with the srad system) whenever he hit 150mph in cold weather, the trumpets would freeze causing power loss.
could be totally wrong, but it's possible in theis weather, especially if all readings are saying normal....