I too was also interested when the stock was initially released by McMillan sadly I already had an A3-5 on order and couldn't change it.
However this stock has not been well received in the us as far as my research has gone as I looked to swap it out. A certain US company bought well into the design, and ended up doing special pricing. I just don't thing that comb was high enough.
The A3 sporter would be the one to look at as a non-adjustable cheek piece option. Although it's fugly.
My sako sporter McMillan is certainly a keeper even when using a 50mm objective. The cast is spot on for me. Shame it won't take a heavier barrel.
"If you can't see it, you can't shoot it"
According to McMillan's specs on the A1-3, the comb height is the same as the A3 sporter.
I already have an A3 sporter, and the comb height for me using a 56mm scope is spot on, but if I am critical of the stock, the pistol grip is too short if you have large hands - like all the A series, and the forend is just too wide at the rear to be completely comfortable when using off hand or in hard hold for a heavy recoiling rifle.
Hence my interest in the A1-3 stock.
i have had a good look at the A1-3, even had a fondle of one attached to a remington xm3, found the stock very comfortable from all firing possitions.
just need to put some money away to get one now.