I note that Patton forbade such augmentation because engines were inadequate. But he was rampaging across-country, whereas the Marines were using them, as you put it, as mobile pillboxes. Was that why they were successful in the Pacific?
I think so, the Sherman was originally designed as an infantry support tank not a tank fighter. That is why it had the “medium” powered 75mm gun. The Army Ordnance Board wanted a cannon with a life span of 1,000 rounds IIRC.
The Marines in WW-II were engaged in island warfare for the most part. Not a place where you were likely to have pitched tank vs tank battles.
Regards
alfa6 ;>}