So far, impressions are very good. The color!!
With the valve on, I really enjoy the response most of the time- however, there's a leak, I think in the slide receiver going into the valve, so I'm losing some energy. There don't seem to be any quirky notes... high Ab feels great. In fact, all of the high register feels great, with defined slots but room to wiggle. Whoever make the F wrap made it quite short- with the slide all the way in it's halfway between Gb and F. To get it to F, it has to be pulled an inch and a half, much farther than any other wrap I've seen. I can just about play it with the narrow slide and Thayer- it touches my jaw lightly and doesn't seem to impede my pivots, unlike the 36K with stock slide. Low range is ok... it works, but I think the leak is not helping matters down there. I may have Benn shave down my valve casing for more seal.
I got a counterweight, so I can play it as a straight horn. In this configuration it's definitely more efficient, with more feedback as well. Ergonomics are still not great- I don't know if stock 8Hs are like this, but the bell brace is a bit far back to hold comfortably with the thumb, and it still feels a tad front heavy even with counterweight. Hot damn if it doesn't play great like this, though!
The slide, despite having probably 25% chrome loss, has amazing action. Not going to change anything about it.
Perhaps most strangely, it seems to play just fine with my Ultimate Brass SMP mouthpiece, which is pretty wildly open in the backbore. I thought it would feel awful on this instrument... but it doesn't.

Not complaining, just unexpected.