They are a hoot!
The 9mm sterling shoots like a 22 and will do 200 yards, but it is heavy
I bought a 9mm Wolf conversion barrel for the G27 and it worked so well I tried a Storm Lake .357SIG and and a 10mm for the G30, and it was fantastic! I can't really tell a difference between .357SIG from 40cal S&W but it may have had a little less recoil. The 10mm was also maybe a little less a hand smacker than .45 ACP but not much. All in all they were hand smackers, I'm sticking with the 9mm Wolf, it was a joy to fire afterward.
It is nice you can turn 2 guns into 5 for less than the price of one. If ammo gets scarce my odds of finding any are pretty good now.