I usually get about 30-45 seconds more runtime going to 16 from 25. There is a noticeable difference in power using 16. Depending how fast your pit stop is will determine if it is worth the time lost. A lot of the fast guys can't make runtime using 16 without leaning the engine near the point of damage. At most of the races I attend I am about 7-8 seconds faster in 5 minutes using 25 over 16. A good pit stop is about 5-6 seconds more than the average laptime. I can run the engine richer with 25, and make more power than running lean with 16. This helps engine life.