Forget the theoretical answers about what could/should be happening, this should answer the question once and for all...

http://www.continentaltire.com/node/3918

I stand by my long-held belief that if you don't turn factory driving aids off (in a safe environment please) you will never learn to drive fast.

 

My mom used to say everyone should learn to type on a manual typewriter. At this point most all cars are loaded up with drivers aids so maybe the best way to learn to drive fast is to find out how to make the most of each of your cars "advantages". You can't turn off all the factory driving AIDS on most cars anyway.

 

Ever tried an autocross run with traction control on? I’ve done it in both a fwd MINI and a rwd BMW, and it was dramatically slower (like multiple seconds on a 40-50 second course, or one time I forgot to switch DSC off in the MINI, and thought something was broken with how badly it was bogging down) - this must come down to the "safety" nature of most street car systems as described in the article. Maybe getting into really powerful and supercar territory it starts becoming more performance oriented and beneficial.

ABS however, I don’t think there’s nearly as big a difference there. Maybe that’s partially because I’m just not very good at threshold braking, but your lap times are going to be hurt much more by coming out of a corner slowly (say due to overly aggressive traction control), than by not carrying speed a few extra feet into a braking zone. I would pull the ABS fuse in my old wrx for autocross, and it was definitely more fun and challenging, but I would occasionally lock a tire, and probably “underbraked” quite a bit too. It felt fast but I wouldn’t be surprised if I would have been faster with ABS on, so I could just mash the brake pedal and focus on my line.

 

ABS is no longer as helpful as it once was. See "ice mode" in autocross discussions.

I once left DSC on for an autocross run on a fast course. I did not need to brake for a single corner (2010 MINI Cooper HS).