A friend of mine has an R9 in .177 with a VMach kit. Just the spring, guide and seal. It's buzz free, and makes good power. But the Maccari kit in mine was buzz free, made the same power, and had less recoil.
And was 60 or so bucks cheaper.
Now the one with the whole piston may be a better improvement, but I somehow doubt it. I would have a custom oring seal piston cap made and tune it to suit instead. I think that would be a much better setup for the money.
I tried both Vortek and Maccari spring and guide tunes in my HW77K 25mm. All were nice. But once I had an oring seal cap made, and slightly drilled the transfer port, it was a huge difference. Just a quick "thack" and a gentle nudge at the shoulder and you've got a 13fpe CPL headed down range. That's with a Maccari GSX OX spring kit behind it, with 2 of the 3 spacers installed.
For me, I can't see spending the $400 on a drop in kit. Id rather spend that money to have a well known tuner do a tune with an oring sealed piston. They can tailor the spring and seal to YOUR gun and what you want it to do. And also change the transfer port if needed. Barrel snugness and caliber should be taken into consideration for the best super picky tune.
IMO, the piston bearings are unnecessary on the underlevers. But buttons on a piston of a breakbarrel can't hurt.
Just thinking out loud and sharing my little tinkerings. It's hard to go wrong with any tune these days. We are lucky to have the options. Better than sending it to Beeman's to have them pack it full of goop!