Many thanks to all for the replies. I wasn't aware of the 54's reputation as a scope killer. I had always assumed that its recoilless operation would be a plus for scope longevity. If this is not the case, what advantage does the 54 have? Its power rating is the same as the 48/52, and it's not like the 48's recoil is a source of any discomfort. Is there an increase in accuracy as a result of the elimination of recoil?
Have the rifle tuned by a pro (Paul Watts and several others can do it for you). An upgraded spring, seal, buttoned piston, and trigger tune make a huge difference, especially in certain Diana models from my experience. The Diana 54 Air King is recoilless to you the shooter, not to the scope. All the more reason to tune it and smooth it out.
Then put money aside to purchase a good quality magnum rated scope with Beeman 5039 mount or equiv. Plan on spending approximately the price of the rifle, what can I say. It's not a cheap hobby but far less money than playing golf, racing cars, boating or dating the wrong woman.
48, 52 or 54, you'll experience what that rifle can do. All this won't make the rifle any lighter but it will smooth it out enough to make it worth the investment if you want a side cocking magnum rifle. The weight helps dampen some of the recoil, just like a Beeman RX or AA TX.
If you want light and high power, maybe a PCP would be a better bet than a springer. Springers have "soul"!
The custom 52 .22 I have shoots very well out to 75 yards. Squirrels hate that rifle. I shoot it mainly off a bench rest. Right now I have a Weaver V16 scope on her but I liked it better when I had the Beeman SS-2 compact 4X21mm on. Far less mass up top side and you can tell the difference immediately.