In regards to the grip, is the hand position similar to a thumbhole?

somewhat similar,..but your thumb will be positioned up on top, and behind the tang. This makes for a better reach for the trigger,..by positioning your hand more verticle,..and thereby relaxing the trigger finger instead of making it "reach" for the trigger.

Benchrest stocks are designed this way.

Does anyone use any McMillan A series tactical stocks? What about for varmint hunting?

Thanks

I have an A2 on a 308 with adj cheek and butt. It has a nice flat bottom for riding a bag but not as wide as say a benchrest stock. Verticle grip may take you a while to get used to,..but they shoot very well.