Some have surmised that a mag primer is worth a grain or two of extra powder. I can't confirm that at all, but it is a fairly safe bet the mag primer increases pressure under most circumstances.

Oddly at first blush, I've read that mag primers can DECREASE consistency in burning of certain types of powders due to the added blast effect forcing powder away from the initial force of explosion before the powder ignites. I also cannot confirm that but it would seem to make a bit of intuitive sense.

Regardless, it bad loading practice to simply replace a Standard primer with a Magnum primer in a max load that was worked up with a Standard primer without backing off 5% and working up afresh and using the mag primers from start to finish.