After owning and shooting the 3 you mentioned, my M1A Bush rifle would out shoot both the Std, and the Socom, but not by all that much, and out to 300yds you mentioned and then some.
FWIW...... My Socom started out as a Socom II, and did shoot better once I removed the rails up front, and replaced them with a std hand guard. The rails are clamped to the bbl (Same as the scout mount), and in my case...had an adverse effect concerning consistent accuracy. ( I for one would be interested in what other M1A owners have found, regarding the scout mount). IE....... bbl harmonics.
As they come from the factory, all have iron sights only. The 3 you mentioned all sport different front sight posts. They vary when it comes to height and width. The difference in height correspond with the differences in bbl lengths. The widths on the other hand have the most effects when it comes to acquiring a target. The Socom has a rather wide front sight post, and while this feature works rather well (in conjunction with this rifles larger rear sight aperture) for close quarter shooting, it makes it very challenging when it comes to the type of actual precision your referring to.
There are other factors to consider when it comes to this type of rifle as well.....with the end goal of said rifle cycling smoothly, and with all moving parts returning to the same spot every time it is being fired....... to make it short and simple.
Sub MOA at 300yrds?
If you are serious about an actual sub MOA rifle then look here....
http://www.gaprecision.net/
George and company can make it happen. I haven't had the opportunity to speak with him recently due to my current job overseas right now, but he has built precision rifles from the M14/M1A platform. Not sure about the sub MOA requirement of yours however. Look at his website above and see what he has. If he says one of his rifles can do it, then it is the truth.
The M1A std/Scout/Socom do not fall into this category. They are built to be extremely reliable/work in all environments/climates/weather conditions semiautomatic "battle/MBR" rifles, and cover this aspect very well from my experiences with them.
FWIW..... The M14 works just fine in the role of DMR at work
If you get one and shoot it long enough to "out shoot your own rifle", you can always look at additional options at that time.
11B