One thing to note is that the reason the Germans went to the larger bullet was because they wanted to deepen the grooves. The bore diameter of the J and JS is the same, .311". They did not rebarrel the 88's, they just ran a new reamer into the chamber to ream out the neck of the chamber and allow the larger case neck to expand. That prevented high pressure that would result if the case neck could not expand to release the bullet.
FWIW, I have fired a fair number of WWII 7.9 in my "S" marked '88, and nothing happened, but that does not mean something couldn't.