I've been following this thread closely but didn't want to post until I had an opportunity to try my new setup. I've mounted a scope on my AR last year for across the course service rifle competition. I've had a lot of problems with eye relief between the three positions shot in that type of shooting, namely prone, sitting, and standing, due to differences in head position. The scope needs to be furthest forward for comfortable prone position, but further back for the other positions.
I started out with a fixed A2 stock. My mount is a Leupold Mark 2 which I assume is a 2" offset. I just replaced the A2 stock with an adjustable (Magpul CTR with the 0.7" butt pad extension) which allows the stock to be shortened for the other positions if you can get it far enough forward for a comfortable prone head position. I finally had a chance to shoot it prone yesterday, and it was more comfortable than previous, but still not 100%. I'm going to try and live with it because I'm tired of throwing money at it.
I received a email from Creedmoor the other day advertising a 2.5" offset mount. I think that would be just about perfect for me. Dennis Demille definitely has his hand on the pulse of competitive shooting.
I guess the conclusion to this rant is that, it depends on what type of shooting you plan to do as to what offset you need. If prone is in the mix, and you are a stock crawler like me, get the longest cantilever that you can. The scope can always be moved back, either thru the rings or on the rail.
I have never had this problem with any of my non-AR rifles. It seems for a bolt gun I can always find a reasonable compromise that allows for comfortable eye relief for most positions. I also sighted in a hunting rifle yesterday that I had replaced the scope on. I shot it on the rifle range prone because the bench rests were jammed. No problem with eye relief what so ever.