Substituting magnum for standard primers is something that has come up in pretty much every gun/shooting forum I'm a member of again in the last 2 years, where some report no difference while others say there is a slight increase in FPS.
Taking a look at the reports where someone actually got out the chronograph and performed a test where the primer was the only thing changed I noticed a trend. For the reports of no difference, the powder being used tended to be one of the faster ones like 700-x or Bullseye, and the ones that reported an increase used something slower like CFP Pistol or HS-6.
I'll admit there really is not enough information to draw any conclusions on this. The tests all cover too many different cartridges and charge weights. It could just be the difference in the charge weights is what's causing this, where a faster powder usually has a smaller charge, and not the burn rate.
FWIW, I've loaded 230gn FMJ over 4.2gn of 700-x and noticed no difference between CCI #300 and #350's, although I did not chrono the rounds since getting permission and setting things up at the public ranges I go to is a royal PITA.