Uriel,
I will only speak generically. The general case does not match every specific case. If I have a 6 inch piston, acted upon by a 1000 PSI gas, it will develop more force than a 3 inch piston, which is under the same 1000 PSI of gas pressure. A .25 caliber pellet has more skirt area than a .22 caliber skirt area (not a lot, but it is there). This would normally give a theoretical edge to the larger caliber (all other things equal). However, you are also demanding more FPE out of the .25 caliber pellet (55 FPE, versus 40 FPE for the .22), which would normally edged the advantage towards the .22 caliber. In neither the .22 or the .25 do you state what weight of pellet you intend to use, which makes any real calculations harder to define.
The last part is the actual valve and transfer port issues. Although I have a very efficient .25 caliber air rifle, it would have less efficiency if I were to try to take it to .357 caliber, as the air demand would be well over what the current valve and transfer port would deliver without significant turbulent loss. Quality companies, Like Theoben, often engineer their air rifles to have optimum air flow characteristics for the caliber they were manufactured for (.177, .2, .22, .25 were options for that model). If your model was factory engineered for .22 caliber, stick with it. If it was designed for .25, stick with it. When they are engineered, the designers optimize for several things:
1. Desired factory power range, going higher on power than they are designed for always results in less efficiency unless you modify valve and transfer port.
2. Desired length of shot string. Frequently reducing power gives you a longer shot string.
3. Reasonable variation in velocity, within a single shot string. Often on unregulated air rifles they increase transfer port/valve restriction to give a smaller variation in velocity.
The webpage for this air rifle claims with a 400CC bottle, you get 90 shots at 30 FPE in .22 caliber. Getting to 40 FPE with reasonable efficiency may be tough. I found no specification for .25 caliber to analyze.
And to state the obvious, heavier pellets normally give you a bit more FPE, at the expense of velocity.
Best of luck.
Addertooth
Post Script Note:
Most pellets have a sweet spot for velocity. Pushing them faster reduces accuracy. On they highly mentioned JSB exact King II pellets, on many (but not all) air rifles, their accuracy degrades over 900 FPS. With some competition shooters tuning for around 860 FPS for the tightest groups. Some of the barrels with different twist rates, and smooth groove designs can get good performance out of the same pellet at slightly higher velocities. Research your pellets and ideal FPS when you are setting your goals.
- in my penultimate paragraph.