I doubt that you or any one here would see a huge difference as the match conditioning is almost the same for both rifles and both can be extremely accurate. However the M14 does posess a slight accuracy advantage because of the shorter oprod and piston arrangement. Though all things considered when you chamber the M1 in 7.62mm any practical advantage that exsisted dispears. as you lose some of the recoil you had with the .30 service round.
Agreed. While there may be something of a perceived difference in favor of the M1A, this probably has more to do with the configuration and condition of the average M1A, versus that of a typical M1.
Given the M1A's commercial origins, they tend to be purchased new, often in match trim, and well maintained. Garands, on the other hand, frequently come from Anniston (DCM/CMP) as ex-military rifles which must then undergo some sort of reconditioning for serious match work. Depending upon how extensive the overhaul is, and how skilled the armorer/riflesmith performing the work may be, these rifles can be (and often are) every bit as capable as their more recent commercial cousins.
Assuming you have two equally fine rifles, it comes down to design characteristics and ballistics. Sharpshooter briefly covered both of these, so I'll not labor the point. I guess the point is that, while the M1A may be somewhat more convenient from a user's perspective, either of these fine .30s is more than up to the task.