Can't see how it was bad advice. Just a little hotter ignition. Never bothered anyway because magnum primers were more expensive and not as easy to come by. SPP's do the job, enough said.
DT
For starters, the magnum primer likely wouldn't add 2 feet per second to the bullet. The primer ignites the powder, but the primer is so instant that the pressure from it is gone by about the time the bullet starts to move. It is highly unlikely that the fast burning powder in the pistol case would be ignited any "better," by a magnum primer than it would by a standard primer.
Actually, I have never been able to detect any difference in a rifle, or big pistol, in whether I used a standard, or magnum primer.
I once did a test. I loaded bullets into the case with no powder, to see how far into the lands the primer would drive the bullet. I loaded a 22 Hornet with a 45 grain bullet and no crimping, with standard, then a magnum primer. Neither primer drove the bullet out of the case.
Next was a 420 grain cast bullet in a 45-70, without crimping. I first tried standard pistol, then magnum pistol, then standard rifle and finally magnum rifle. Not one bullet was shot out of its case!
I went to 30-06, 150 grain. Finally, a magnum primer drove it out of the case, but it did not stick in the lands. I only tried one of each and I was using well used 30-06 brass, so the one that went out of the case may have had a neck that didn't fit as tight.
Yes, it is bad advice to try and increase the power of a 357 magnum by simply changing to a magnum primer.