We are working on a direct drive version now, it should significantly quiet both the Armjet and J-DPS.
I agree, you are not going to see three knots in scuba gear. It moves me (5'10", 215 lbs) in just a mask and snorkel at 200 ft/min (2.3 mph) in the pool. In a good warm water scuba rig with great trim maybe you get 1.8 mph. Still, that is faster than most divers can swim for an hour, and you won't burn all your air, increase your N2 load, and be exhausted. It is also a lot of fun, especially with a light or HD camera in there.
We are also working on the non-adjustable arm ring. If you look at the current pictures on our website (jetboots.com) you'll see the pool shots have a neoprene pad in there. We're going to add some type of adjustment here, maybe as simple as giving you a bunch of dense foam shims that screw into the ring. There is a surprising variation in forearm size between people, especially with exposure suits on! Anyway, the production version will have a solution that allows customizing the arm ring fit, as this is important to reduce arm strain.