A spring piston airgun is already pretty quiet. Way quieter than a Sheridan. A Sheridan with 8 pumps is a noisy beast, and even with 8 pumps they're not particularly powerful at about 13 foot pounds. Pretty much any spring piston airgun will be quieter even ones without cans.
Unfortunately, you can't make a springer that much quieter with a can since about half the noise comes from the action. It won't sound quite so much like you're shooting however.
If you really want a quiet airgun the thing to do is to silence a pneumatic airgun. Whether it's a PCP or a multipump, they respond very well to cans and can be nearly silent. With a full charge, the hammer falling on my S410E makes more noise than the discharge does.
Silencing airguns is a legal gray area. The general consensus is that so long as the shroud is permanently attached that everything is legal, since airguns are not legally considered to be firearms by the feds. Airguns like the Whisper have been legally imported into the states for several years now and the ATF hasn't made a fuss about it. In fact I know that some companies have gotten the ATF to certify that their shrouded barrels were legal before selling them. If you can remove it, then arguably you have a silencer and must get a tax stamp for it, although some airgunners argue this point and say it's still no different than a small engine muffler and that so long as it's not intended for a firearm and not used on one then it's still OK.
If you can remove it and you have a firearm it could easily be fitted to or they were to find powder residue in it, then there's no gray zone left anymore and you've got something that could land you in Club Fed.
But I agree with everything else you say.