Peak temp measurements on the rotors at Laguna Seca showed the ducts alone were worth about 250 degrees of cooling. Turn the fans on and you get about another 100 degrees. Average speed on that track is around 75, with peaks at about 130 on the front straight, 110 on the back straight and close to 100 in the straight...ish part after T6 when you are going uphill towards T7.
Since temp went down when the fans went in, I think the answer is 'not yet' to the choke point question. I'm not certain its relevant to categorize the question into low speed vs. high speed, as you do both. Especially on that particular track at say T11 where you go down to about 20 to make a 90 degree left onto the front straight where you blast off ... straight into a downhill that heads into a slow hairpin. You get goth low and high speeds.
Doesn't mean that the fans aren't in the way of smooth airflow... I think they do as well as they do in part at least due to the meandering you have to do to route them, which doesn't do any favors to airflow. There's no way to do a straight path without some serious surgery into the fender wells I think. Probably would affect the car's streetability if you did that, too.
Page back a little and you will see MikahB recently had a fantastic idea to keep the fans in play but get them out of the direct airflow. I can't figure out how it'd fit yet.
I personally doubt it. I don't see any need. Street driving at even the most extreme is well within even stock brakes' capability unless we are talking about RTs. Even an RT has good-sized brakes that probably need nothing more than better pads and slightly better fluid to be ready for the steepest mountain descent. Add stainless lines to that and you have an good poor man's big brake kit.
Unless you are the manufacturer of brake pads that can't do their job. In that case blame the brakes none of the other pad mfrs seem to have a problem with