also consider that abs is the second or third most assisting... assist of the game, behind SRF and TC.
not only there are hidden stability assists if you use it, which has been discussed thoroughly on this forum, but consider this: braking is the most important part in racing.
If you turn abs on you're missing a lot, especially if you are considering trying more competitive and advanced games/simulators than gt5, or real life racing. In both abs is rarely allowed, and even then you'll end up locking the tyres if you brake like in gt5 with abs 1. Besides, on those 2 cases it's faster and more consistent to break without it.
Of course if you disable abs you'll need to tweak the brake balance, but you can do that on the fly, and this is assuming that you have a wheel. The ds3 has hidden assists as well, and its buttons/triggers aren't the greatest to have total control.
and about lap times in gt5, I'd say it's not conclusive. Check the time trials times: some top10/20 people use it and others don't.