I'll chime in and say that although .177 is the dominant pellet by far, my BAM B-26 in .22 gives about 700 FPS with the CPHP pellets at 14.3 grains.
I had a Whisper that could do near 1,000 FPS with an 8 grain pellet like the Beeman FTS. This gives you something like 18 FPE for the Whisper. It had a Macari spring and a turbo tune, the stock gun cannot do this.
My BAM when new with a stock spring was above 700 and still close to that with the heavier pellet - 15.6 FPE, so similar in close range energy.
You have advantages in that the more massive pellet retains its energy at greater distances, and more importantly, at the lower velocity the ballistics are not so critical. My Whisper was very pellet sensitive, the Superdomes shot a different pattern than the FTS and the CPHP's or the Gamo Hunters. My B-26 is not very sensitive. It does just fine with the cheap pellets, and not too differently with any dome pellet I have tried.
Cost - I find that the CPHP pellets in .22 are about as good as any. They sell in tins of 500 for $8.25 at Pyramid. Buy four, get one free, so something like $30 for 2,000 pellets. Cheap but good. The high quality .177 pellets are way MORE expensive than these.
And regardless of the calculated numbers, my experience (mostly shooting at pellet traps in the 20-30 yard range) is that the BAM would clank that trap (the little Gamo) and put dings in the steel far exceeding the Whisper.
All that, and the high performance springer will require a lot of technique and refinement in scope mounting, etc that is not necessary for the lower velocity rifle. This all replicates for me with the 750 FPS Slavia CZ 634. Months of adjustments and practice, pellet trials, scope and mounting swaps, etc. did produce some results with my Whisper, but the first time I shot the Slavia, I got a better pattern than any of perhaps 30-40 efforts with the Whisper. I'm very happy with the Slavia, but it has about 9 FPE. Fine for my plinking purposes, but if I get mad at a grackle, that B-26 is the choice for me.
My new (to me) single pump pneumatic rifle pushes a .177 pellet at 575 FPS. That's just 6 FPE, but accuracy is amazing if you want to put pellets on the bullseye.
I saw Johnny Quest shoot his .22 Whisper at the NTAGM shoot, and it was very impressive. He did have Mike Melick tune it, and replace the plastic stock. See:
http://www.gatewaytoairguns.com/airguns/forums/thread-view.asp?tid=10516&posts=5&highlight=whisper&highlightmode=1#M115468Watching his impressive results at distance, I think I would still have a Whisper if it had been a .22, although Randall had an adventure making it work this well.
My first rifle was a .22, and I think if I had to reduce to one, I would keep the larger caliber. This is probably not the most popular conclusion, but you might give it a thought.
p.s. the "quietness" of the Whisper is largely a marketing concept in my experience. Maybe a little improvement, NOT dramatic.
