Best velocity for .25

Depends on the gun, the barrel, the range, the regulator setting you shooting at and the pellet grain and brand. My cricket shoots best in the 920 to 940 fps using JSB pellets. Some .25 Airguns shoot better at 900 to 920 fps. Each gun and pellet combination is unique. It takes a chrony and some testing to be able to tune the gun to its optimum velocity and find out what works best. The shorter way to get to this is to search the web for what other users are claiming as the best velocity for the pellet and gun combination that you have. That's a good starting point Chris. 

If there is a 500 cc bottle it would probably give you 60-80 shots shooting JSB 25.39 grain .25 cal pellets at 890+FPS, gun must be regulated like the FX wildcat. If only there was a gun that could fling pellets just under the sound barrier(1200 FPS roughly) while remaining accurate, have high shot count, include tons of sound baffling systems, and a high shot count by having a 500 cc bottle with max/normal fill of 3600 psi instead of 3000-3200 then I think lots of people would buy it. On to your question though as long as the pellet doesn't exceed the sound barrier it won't make that loud crack which is good! However if your looking for high shot count then I can say that my FX wildcat filled to 220 bar with a regulated 300 cc cylinder flings JSB .25 cal at 890+ FPS gives only 40 consistent shots.

I set mine at 880 fps and do quite well in shot count and accuracy with the JSB 25 gr pellets.

If power is your thing, then 980 fps will give you more zip, but 40+ fpe is plenty for me to shoot anything close enough for me to hit with any regularity. Plus, that 100 fps difference equates to quite a difference in shot count. For me it was an additional two magazines before going off the regulator. Faced with a 70 mile round trip for air fills I certainly want to make every shot as efficient as possible.