I picked up my YHM R9 yesterday.
Just got back from the range today.
First tried it with the supplied 1/2-28 direct thread mount with the factory 13 lb spring. With some Santa Barbara NATO 9mm surplus ammo it ran 100%, little bit of blow back in the face. Maxx Tech 124 Gr ran about 80% and Federal 150 Gr Syntec would't chamber another round, but was very quiet.
The started trying lighter recoil springs, went down to 10 LB's and the Maxx Tech was over 90%, the Syntec got a couple of shots off in a row, maybe 10%..
Then I went back to the factory recoil spring and put the booster on the can. The SB NATO was still 100% but felt like standing in front of a sand blaster. The Maxx Tech and Syntec were 100%. No blow back on the Syntec. I only have a couple hundred rounds of the SB NATO left, and the Beretta is my only 9MM that really likes it. (It's chokes my Glocks and Sigs. also they have light primer strikes, it is great for malfunction training though) So I'll use the direct thread with that ammo and use the booster with everything else.