I've got a 2.5" class V hitch on my truck. It came with a class III 2" adapter. The adapter is a PITA. It works fine, but between the hitch, adapter and ball mount there is a whole lot of banging and clanging going on. I only use the adapter when towing my utility and quad trailer. My boat (10,000lbs) gets the 20,000lb 2.5" mount and 20,000lb 2 5/16" ball
And yes you typically lose some capacity. A 2" tube steel ball mount is generally good for 5000-6000lbs. A 2" solid steel ball mount is good for up to about 10,000.
After that you get into 2" mounts that you'd use with a weight distribution hitch.