I have never been fast, so wouldn't know how to speed it up with an underlever, and have not used one in competition. I don't see any big problems, but it would take two hands to drop the barrels, and they drop FAST! Closing needs a hand on the hot barrels to lift them up - then twist your left hand to close the lever. Look back to posts by WOLFGANG. He used a Husky chopped to about 20" bbls. in competition (I think?), that might ease the shock of barrels dropping. There is nothing to resist gravity.
My CAS piece is also an m51 in 16ga. No reason for not being as fast as you can be. It would normally be short chambered. Technically no real reason not to stretch to 2 3/4" but why? What gauge?
"Husqvarna"