Hi Dave,
Spring piston air rifles jump a lot before the pellet is out of the barrel. This suggests that any device that modifies the barrel harmonics can both improve it, or make it worse. So, yes, an appropriate barrel tuner should be able to shrink groups on a "springer". This assumes you that you use an appropriate and consistent hold, else that could be the largest factor in shooting well.
Generally, springers shoot better when you use a very light hold, although that is not absolutely true for all springers. A consistent hold is good for all types of shooting.
An air stripper is basically a vented baffle, mounted a short distance in front the barrel muzzle. Its purpose is to redirect a large percentage of the air leaving the muzzle, away from behind and around the pellet. That is to reduce turbulent air from around the pellet, to reduce buffeting of the pellet, and thus to improve grouping.
The way spring piston air rifles work, they do not have much muzzle blast. This is because the volume of air they compress is not very large; especially for lower powered versions, such as the HW30. Piston bounce is another reason why muzzle blast on springers is less than for PCPs. But mainly, it is that very few practical springers produce more than 20 to 30 foot pounds. Whereas, PCPs over 12 foot pounds are very common outside of the UK. So, air strippers are more useful on more powerful air guns because they have more muzzle blast. So, much more useful on PCPs than on typical springers.
Mostly the concern is about "accuracy", or precision - or it should be. That drives the avoidance of shooting common round nose pellets over about 900 FPS in any given caliber (wadcutter pellets shoot better at much lower velocities). This max practical velocity threshold drives the "practical power limit" for each caliber. Yes, if you shoot heavier slugs they respond well to being driven over 900 FPS, generally up to about 1050 FPS. Pellets tend to spiral at ranges over 50 yards, if you shoot them too fast. Too fast is over about 950 FPS, for waisted pellets.
An air stripper has some mass, so attaching it to a low powered springer may tighten groups. Not because of turbulence reduction, but because of a serendipitous improvement in barrel harmonics. By the same token, mounting an air stripper could open groups due to driving less favorable barrel harmonics. All that said, break-barrel springers have heavy barrels, to act as cocking levers (compared to commonly thin PCP barrels). So, adding a dainty air stripper to a springer is probably going to do very little, either way.
