So I got around to shooting a string of 100 at the minimum setting (1/4 from stop on the regulator), which seems to be right at 1700. I wan't all that happy with the 10 shot grouping, but some of that was probably me (the prior experiment seemed to show better grouping at ~2000). I had the chrono out this time though and have the data:
I had a couple of shots first shots that the chrono missed (the blanks in the blue line). Other than those two, I did see the first shot bump again at this pressure (regulator pressure appears to creep up during the delay while I reload, save chrono data, and record the gauge pressures). I didn't time my stop this time, so it was probably between 1 and 2 min between each 10 shot group. The FPS spread and std dev is again worse at this minimum regulator setting compared to ~2000 psi. There continues to be a consistent drop in the reg pressure as the fill pressure drops. In this case, before the fill pressure got down to the original regulator setting of 1700 (took 94 shots), the regulator pressure had dropped to 1450. This appears to also be evident in the downward drift in FPS.
I figure I will try a regulator pressure setting between the minimum (1700 psi) and the 2000 I went up to last time. I'll keep track of the number of turns on the regulator screw too.
Still no response from Air Venturi about my questions. I sent a message via their website before Christmas, and multiple messages left on their phone since then... I'm trying to decide if I want to pull the regulator to check/clean it in the hopes that might help, but wanted to chat with them Air Venturi first.
As a note for people reading this, Don't think I am angry, I am still happy with my Avenger purchase. I just think it can be better (as could the responsiveness of Air Venturi). This is my start in PCP's, and if there wasn't something in this price range (Avenger, Gauntlet, etc), I doubt I would have jumped in if I had to start at $800-$1000 or more for a first PCP.
Edit - Also, if anyone is interested, the Tuxing compressor took exactly 8 minutes to fill from 1500 to 4300. Not sure how accurate the gauge is, but it started at 22C and got up to 37C in that 8 mins. Its loud, but so far I happy with it (I swear the fan is louder than the compressor, or at least more annoying of a sound - it sounds like a jet engine).