Now, after all that. If what Speer is saying is correct, and if in fact, GI 30-06 and commercial 30-06 of similar weights are really very similar pressure wise once you get the indicator correct, I really dont see a problem here.
Now, if you dont buy into all of that, and are just going to base it on velocity alone, I seriously doubt that they would design the M1's operating range with a margin of error in that was as narrow as 100fps, give or take.
I think you are missing something.
First off, modern computer tech allow for a much more precise pressure measurement than the old copper crusher measurement system. In fact, some loads that were measured as "safe" under the old systems are now considered over pressure using modern measurements.
Another thing is that the SAAMI specs are for commercial .30-06 ammo, not the GI ball. Yes, similar bullets at similar speeds will have similar pressures, but the SAAMI commercial ammo loaded to 2900+fps is NOT the same as the GI ball loaded to 2750fps. 50k pressure for GI ammo, 60K for faster commercial stuff sounds right to me.
Also, the same situation exists with the 7.62NATO/.308 Win. Commercial ammo is hotter, and loaded to higher pressures. Remember that the 7.62NATO was loaded to duplicate the GI .30-06 loading for velocity.
With the Garand, its not 100fps+/- that makes the difference, it is the pressure curve (pressure/time) and specifically gas port pressure. The rifle will work over a reasonable range of pressure & velocity combinations, BUT some of them will work the action more violently than intended, due to higher gas port pressure at the critical moment. Overall load pressure might be within allowable specs, but if the pressure at the port, at the critical time is too high, its hard on the gun.
Also remember that the Garand was designed to run on GI ammo, something with a specific set of specifications. The fact that it can operate a bit outside this range is nice, but was not something intended for in the design.
Think of it a little like a car engine (not a perfect analogy, but close). This engine is made to run on 87 octane gas. You can run it on 100 octane, for a while, but things are going to go wrong at a higher rate than when you run it on what it was made for. And if you put 150 octane in it, bad things are going to happen pretty quickly.