There could be. Everything being set up as equal and perfect as possible, you should be able to get about a minutes extra of run time with a .21 over a .25.
That being said, how often do we ever get "perfect" anything....

For the average joe racer, maybe about 20-30 seconds. Is this a lot.... Could be as much as a lap at the track. Also could be the difference of a 3 stop race and a 4 stop race . it you are running a 30 minute main and can make 7 minutes with a .25 and 8 with a .21, you can make it in 3 stops with the .21. Now this is just a ball park.

Engines today are so over powered as it is, I prefer to be able to control it. Races are won on the track. But they can be lost in the pits.....