Randy, what weight dead center? I ask that because, I am thinking my velocities with a 300 grain XTP is another 100-150fps faster... I think I am using the same chrono you are - maybe elevation I am shooting at 2700 feet elevation. I am at work so I can not really check the velocities I have recorded.
Have you tried the MMP sub-base with a 100 grains t7 yet? I have some but have not tried them yet to see if they make a velocity difference. The sabots that I recover really do not look terribly stressed at the base, with 100 grains of t7.