KMS kills 2D performance all across the board. However, it's not that bad with older chips (my 9800pro and 200m for example, and I would guess any pre r5xx chip as well); with those it remains rather usable. With my hd4850 KMS really screws things up (I will assume it's the same for any r6/7xx chip) as it is much much slower than UMS and certain applications are just unusable (Firefox being one).
Maybe that's because 2d runs on the 3d-engine on those newer chips?
With my x1900xt 2d seems very snappy, but I've only tested KDE-apps, not firefox or anything else.