Don,
I'd say this: If a bench can work at all well using good technigue with a centerfire rifle for long range shooting, it CAN work even better for airgun benchrest shooting. As many of us realize, long range outdoor shooting is a wind doping game, mostly. but you DO need to be able to aim precisely where you want and break the shot when you are on aim. Of course when a centerfire rifle goes off, some of the commotion it creates CAN affect the shot, so extreme repeatability of the shooting technique is important for good accuracy. I KNOW a good portable bench that can deliver decent results from centerfire rifles can deliver good results from airguns, because most competitive airguns really don't disturb the setup nearly as much during the shot cycle.
There is a club that shoots almost every week in San Diego at a police pistol range. Though they are allowed the use of the range, there are no benches, and they are not allowed to put any up permanently.
There is a series of long 2"X12" boards at hip height delineating the firing line. Shooters generally place their guns and ammo on that board and shoot from behind it. So, some of the airgun club members got together and made some plywood tabletops with (two) folding legs. They attach the front of each table to the underside of the 2"X12" board, and the two legs hold the rear of the bench up. Though it doesnt take a lot of time to set up and tear down the benches, they are really not very stable.
I have shot at that range a number of times, and, after the first few matches, I began to bring my own portable, because It was so hard for ME to shoot good scores n the two legged ones. See, I found that by simply pushing a bit with my arm laying on the bench I could engage more than one bull on a card in either horizontal or vertical deflection. I also noticed that if I shot next to Howard, his rather violent loading motions could physically disturb my aim.
BUT ... here's the point, and its a very important one: These folks are enjoying their regular weekend matches, which are fired at regulation distances on regulation targets, including the rather difficult International 25M airgun one, as well as the old BR50 target, which is perhaps MORE difficult, and, were it not for the temporary benches, they wouldn't be shooting there!
And there's this: Even though it's usually a bit gusty, and there is some tricky swirl due to the layout, there are good scores being fired there all the time! Charlene Pray essentially dominates the standings there, and can shoot alongside the best airgun BR shooters in the World and hold her own. Roger Lovett does real well, nipping Charlene's scores quite often ... I have shot there and done fair myself. We ALL use temporary benches.
To be clear: It would be foolhardy to say ANY portable bench could be BETTER than a really good heavy, well-designed permanent bench, but for sure decent results CAN be obtained from good portables.