Okay, got to shoot it today at last. The good news, it's a ton of fun. It's actually quite loud which adds to the effect. It was also reliable. I have the MP40 as well, and the magazine on it has an annoying tendency to not push the BBs up fast enough for it to fire. I did use lube on it but that only helped a little. The magazine on the Grease Gun on the other hand fed ammunition well.
Having said that though, my luck with getting Umarex guns that shoot way off from the point of aim continues. It shot about 4 inches to the left at 30 feet. I did unscrew the barrel tube and rotated the barrel to see if that would help, but it didn't. However the rear sight, while technically not adjustable can be removed with a screw so it looks like I can modify it to correct the point of impact. I have a Legends Thompson that also shoots off to the left, but I haven't figured out a way to modify the rear sight on that as of yet.
The only other issue encountered is that the magazine fits very tight into the well and is hard to seat and remove, but I suspect that it'll wear in.