Puddin.. our son has installed this on his Catalina 36 to good effect. I considered it for our own boat but didn't need to use winches so stayed with our cockpit traveller with mainsheet cleat attached (plus a fine tune)
Most mid boom (cabintop) setups on 30 plus end up using a winch, seems like, so the speed advantage is already lost. I suspect you'll see more of these setups for the reasons mentioned... more free space in cockpit and getting sheet to the helm.
On the Niagara 31 the double sheet setup is near the end of the cabin top, a couple of feet further aft than your typical cabin top traveller. I can't recall if there's a bridge deck on the N31 cockpit but that would be a good place if you can live with the intereference with seating and access to companionway. End of boom would probably put the traveller across the cockpit seats in front of the binnacle... again, choices to make.... are there seat hatches in the way.... etc etc.....
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)
1984 Fast/Nicholson 345 "FastForward"