I'm not a Pershing expert by any means, but I do know that the reality of the Panther is not what the modern fan boys pretend it was. I also know that in AH one of the best tanks is the T-34/85, at least among those players that are really good with tanks. It is not without problems, obviously. With that in mind when you read about the kill ratios of Pershing tanks in Korea versus the T-34/85 (albeit against crews that were not as well trained), the Pershing most certainly looks better than your comments imply.
The Panther sucked in WWII. It had great armor, a proven gun, but it could not operate for more than 150 miles without destroying itself. Even the M4 could manage ten times that, and I see no reason to think the Pershing breakdown rate was any worse than the M4.
The real reason to add the M-26 is for the "Super" version of the tank. That tank could penetrate Panther armor at 2600 yds. Of course, it would be a perk whiners nightmare.