My Airmaks Caiman is a bullpup that costs about half what an Impact costs and has a pretty good shot count. The huma regulator is not externally adjustable but that is not a big deal for me. I have adjusted most of my guns but only until I get it shooting like I want it to. Then I leave it alone. The Caiman airtube is almost 300CC (I have the longer X model) if I remember right and can be filled to 300 bar. Even at the 250 bar I normally fill to I get around 70 shots with 18 grain pellets flying over 900 fps from it. They make it in 25 caliber too and the internal passages are pretty good sized, I would be surprised if the X cannot make 60 fpe in 25 caliber, probably with about 50 shots.
But Airmaks makes other airguns too including a Krait with a bottle and looking more like an Impact.
But if shot count is your only issue with a Zelos either buy a M60B with a bottle or buy a Zelos and a bottle conversion kit. I think you can even swap full bottles on. The Zelos has a little longer barrel so if you want the maximum power the converted Zelos seems like the way to go.
Personally if I wanted a higher powered 25 caliber I would look hard at the Remington Spirit X or the P35X. The shot count will not be real high but they have relatively long barrels and 60cc plenums so they should make great power. Wood stocks too.