Haven't ridden it yet. Had 150x30 with no stacker, now it's 158x30 with a 2mm stacker. Gives me 5mm more reach and is 8mm higher. The 8mm felt very good track standing, will have to see whether it's still good, when it comes to real riding.
It is a lot stiffer than the Rockman though.
Edit: Just came back from the first session with the new stem. Even though it's not that much longer, it feels better when gapping as there seems to be a lot more leverage in the 5mm more reach. Other than that I didn't feel much of a difference. It is more comfortable, that I can say.