I have not. But I've heard a number of Dynalo builds, Dynhahi's, and a GS-1. Have you heard a Master 5/6/8/9? They are all known to be excruciatingly neutral, 'dead nuts neutral', wire w/ gain sound. That is also the sound that GSX owners have been clamoring about now forever as well. Dubstep Girl recently compared both-and found them nearly identical. If you read between the lines at her experience (w/ Mytek pairing)-you would also likely conclude that the AGD is actually more neutral if anything-but either way they are almost identical. Kevin Gilmore is also extremely impressed w/ the Master 8 design based on it's topology, implementation, and claims he couldn't even source those same parts for $1500-let alone build it and sell it for that. ;)
In regards to the Master 10 being possibly just around the corner-yes, more than likely. But keep in mind there is very little difference at all between the 5/6/8/9 as they stand-just slight power increases, and in one instance the simple implementation of a hi/lo power switch on the back.
I owned the M8 and it is absolutely, unequivocally the most neutral, transparent piece of gear I've ever owned or heard. To my ears it was a good step up over the Dynalo/GS-1 I had previously owned-by a fair bit. Your own comments on the GSX mkI w/ upgraded modules tells me you've experienced the same.
So-heck yes I'd absolutely consider the Master series on equal footing with the GSX mkII-and it seems I'm not alone with this. Sure the aesthetics of the Headamp are untouchable-but I'd wager a lot of people here wouldn't find that worth an extra $13-1500 or a year wait time. ;) I also agree that an RMA to CHINA would be a REAL pain in the butt. OTOH-you could buy a second one with the money saved (bada bing!). AGD actually does have a decent warranty program though-details on their site if you're so inclined.
Don't get me wrong-I'd personally pay the difference for the aesthetics alone, even though I actually like the looks of the huge AGD stuff. But the GSX would be an amp I keep for LIFE (like my Leviathan)-and to that end paying a premium for aesthetics for the next 40 years isn't that big of a deal to me. But dang it the wait time is lol.