Ah, looking at the mouse driver was a good all; I started fooling with it and it seems I experience a problem with high polling rates. At 250 Hz sensitivity behaves as expected, but at 500 Hz I start experiencing small amounts of negative acceleration and slightly decreased sensitivity. At 1000 Hz, which is what I was using before, the effect becomes so extreme that the sensitivity appears much lower than it actually is.
Thanks for looking into that. Sorry for wasting your time.