What cartridge were you referring to with the fed and 2400 combo?Well at least in my testing with federal 200s, there was a significant reduction in fps with 2400(70). There was also a reduction with AA5 which is a ball powder but the difference was only about 20fps. The magnum primer was still slower. I believe the federal must be "magnum" only in cup thickness.
I'm also a fan on doing what the book says but there are plenty of cartridges out there where primer selection is not critical. I've been running sr mag in 223 loads for a while now and I'm seeing almost no difference throughout several loads versus standard primers.