I have never had a firearm that gave velocities far slower or faster than expected, so I've never been able to experimentally test the question.

I've seen a Sierra tech rep express that you can add powder until you reach rated MV.

That's probably dangerous because with some loads, MV reaches a point where it does not increase with more powder. That happened to me with a 308 and 2520 powder.

Be not weary in well doing.