I’ve just fitted one to mine, 10 speed drive train with the sunrace 11-42 cassette, before I fitted it the shifting was fine, (B-Screw pretty much as far in as it would go). However in the smaller couple of cogs, when any power was put through it would keep slipping. (It was a new drive train)
When you actually look at the mech in these gears the top jockey wheel with the B-Screw in was so far back the chain only wrapped round the top few teeth of the cassette. I fitted one and it allows you to have the adjustment on the B-Screw to bring the top Jockey wheel further forward, enabling some more chain wrap on the smaller cogs.
Which after one ride seems to have fixed my problem with the chain slipping. I wouldn’t say it has made any difference in performance of the actual changing gears up and down though.