Personally, I think improvements would still only be limited. Their true enemy is still an evil businessman who wants to get "stupid rich" by hurting people and Shredder is basically just his attack dog. Splitting it into two characters may have left out this Shredder's having any reason why he should even give a crap about Sack's scheme or the Turtles (would it have been so hard to give him some kind of stakes in the matter?)... but on the other hand, even if Sack's remained Shredder, it still doesn't make sense because he has NO clue that his so-called "cure" would for some inexplicable reason mutate his lab animals into intelligent humanoid people, doesn't question it, and is completely unsurprised that it even happened. (Sure, he was mildly impressed, but definitely not the right reaction...)
And instead of changing course because the new mutagenic revelation SHOULD have kind of interested him... no, he still just wants to make the city sick to get rich while he STILL also hasn't any idea if the mutagen will cure anyone. Will be a bit hard to get rich if you just end up killing everyone instead.
Meanwhile... yes, "even if it kills them"... lets just murder one of the most massive discoveries in science history that could have made you rich and famous.
Harming the citizens of the city - including perhaps killing some of your clientele/business partners/etc
who give you money - is so much more attractive then telling the world you found a way to turn animals into people.
Sack's doesn't need to be Shredder to still not make sense, he did a pretty good job of that himself, too.