NOx provides a big power boost to piston engines when used properly. But there is no "free lunch". Racers have used it successfully for a long time. Most major racing organizations have banned it's use in order to help teams control costs and equalize competition. It is costly for all of the above cited reasons. I seem to recall that car racers got the idea to use NOx from some WWII aviation applications. Don't know what types. The only current aircraft use of NOx I am aware of is in unlimited air racing. I don't know how many air racers use it other than "Rare Bear". Race 77 is a highly modified clipped-wing F8F Bearcat sporting a nitrous guzzling R-3350 radial making 4,000+HP. Like all the top racers, It has had it's share of "maydays" after suffering engine trouble during races over the years. A sight, sound and feel to behold! Here is a link to the bear.
http://www.rarebear.com/
Best,
Westhawk