I've chronographed a lot of loads using both standard and magnum primers. The results are mixed. Sometimes it makes very little or no difference and sometime it can change things a lot.
What you need to do is shoot your loads over a chronograph. Look for a change in velocity but more importantly, the velocity spread from highest to lowest. The purpose of a mag primer is not to increase velocity but to get better ignition so your powder burns more consistent. Mag primers do produce slightly more energy so you may have to back off the powder charge a grain or so. Generally, the slower the powder burn rate, the more need there is for a magnum primer.
I always recommend using a powder burn rate chart in conjunction with a good reloading manual (or 2). See: http://www.varmintal.com/pburn.htm
Rifle powders start at #58. The higher the number, the slower the burn rate.
The case volume has a lot to do with burn rate too. A powder that would be considered slow burning in a small case might be way too fast in a large case. Your reloading manual gives some good clues too. Look at the listed powders for a given bullet weight. The more powder it takes to generate the same velocity, the slower it burns.