Air rifles are kinda like centerfire rifles because they are all unique, but generally follow trends. That being said, maximum energy and accuracy generally occurs at 850-900 FPS.
If you know the energy with light pellets, use that energy and set an arbitrary speed of 850 fps to mathematically derive a pellet weight. Top selling 22 cal pellets are 18 and 15 grains for a reason. Like you said, fast light pellets are like heavy bullets in a slow twist 223, bound to be disappointing. For most airguns, under 900 fps is better than over 900 fps.
http://www.airgundepot.com/airgun-calculators-article.html
