If cost isn't a factor then Leupold wouldn't be in the list unless we are talking MK6 or MK8.
The max effective range is probably 800 yards or so.
What do you plan on hitting at 800 yards?
A 2moa red dot is 16" at 800 yards, well within a normal chest style sniper shot.
The military usually runs a 10x fixed power scope on similiar rifles.
I personally prefer to over scope my rifles and would go 8-24x-50mm ffp if money wasn't an option.
The big thing with M1As is the scope mount. Spending money here is more important than the scope itself. It needs to be solid for repeatability.
Here is a video of long range hog hunting.
http://m.youtube.com/watch?v=7UQTQpEUni0
Mk6 or Mk8 seem like a good option. Especially since it can be moved to another long range rifle.
Any scope can be moved to another rifle, does not mean you will not have to go through the re-zero process each time.
If you are shooting long as suggested by the op the mk4 is lacking. The turrets absolutely suck, no zero stop, only on option for turret reticle correctness and they only offer a few reticle options. I wouldn't buy one unless you are getting a mil/leo discount. I have one and the most positive thing I can say is the glass is pretty darn good.
Kenton offers a zero stop knob for the Mark 4. I have one on my .308 and it works fantastic.
I personally have no issues with the turrets but some may.
For the average shooter IMO the reticle options are sufficient and there are some new options for 2015.
I guess if I had one beef with them is that they primarily offer MIL/MOA instead of MIL/MIL or MOA/MOA. There are FFP options that can be had with MIL/MIL but the price jumps significantly.
Btw I do have a Mark 6 and it is a fantastic scope but well on the upper end of the price range for the options offered.