Just imagine how important it is for a cropduster, not only does a change in density affect engine power and tuning, it also affects how the airplane flies in the air.
Early in the morning when it's cool I can haul a full 300 gallons, during the heat of the day I'm lucky to take off with 250 gallons.
Before the new engine, it was 225 in the morning and 160 in the heat.
Big difference, especially when you have powerlines on one side of the runway and a pecan orchard on the other.

On the subject of airplanes and radials, here's another alternative to NO2...water/methanol injection!
It's been used on all sorts of aircraft in lieu of NO2, but without quite as much power gains.
You can expect to see about a 10-20% increase, depending on how close to the edge you want to get, so for a 50HP engine you'd get about 5-10HP increase.
Good thing about MW50 injection is that you can use it in conjunction with NO2 if properly tuned.
Here's a great thread about it:
http://forums.nasioc.com/forums/showthread.php?t=937176

I guess a guy could make a completely new engine with ultra high strength steels and titanium alloys, then pour nitromethane, NO2, and MW50 in until something explodes