The magnum caps do produce higher temperatures and slightly more hot gases, I don't know if they
cause more nipple erosion than the pressure and gas cutting that's caused by the powder charge itself.
On page 83 of Dave Ehrig's book "Muzzleloading for Deer & Turkey", he lists some information on percussion caps and primers.
#11 standard cap - 6.53 cc of gas at 3,024 degrees F when fired.
#11 magnum cap - 7.59 cc of gas at 3,717 degrees F when fired.
U.S. #2 musket cap - 14.36 cc of gas at 3,717 degrees F when fired.
#209 shotgun primer - 21.98 cc of gas at 3,024 degrees F when fired.
#200 rifle primer - 11.68 cc of gas at 3,024 degrees F when fired.