WeirdWorld will probably end up remaking old art and common sprites at a later date. Right now, it seems like they'd rather focus on moving the game forward, and probably come back once the game is over to update and clean up what needs to. It's actualyl a really smart way to go about it, since there are plenty of devs who decided to spent multiple updates just doing replacing and cleanup who ended up losing a lot of support because the game itself wasn't progressing.
Plus, honestly, if you push past the first 15 minutes, the quality of the story works for itself : I, myself, got put off the game twice because of the art. I ended up caving in and trying it because I was bored, between games, and wanted something to play. And the story hooked me so hard I couldn't put the game down, and even ended up getting used to the art to the point of liking it.
Honestly, if WeirdWorld ends up remaking older scenes like I suspect they might plan to, I really hope they leave the old art in the gallery at least, maybe with a button to switch between the old and new to show how far they've come on their artistic skills. And to not just make the old art dissapeared. Sure, it might be janky at times, but it's honestly got its own unique charm and sexyness.
I could also very well be misremembering. I'm not like Drako, I haven't played the game 10 times already.
Going for corrupting actions does give you a lot more power, but you can avoid getting locked out of some content without it as well. It's just a bit more complicated since you need to be careful to hit every possible point gain.
As a rule of thumb, always do all the training possible, and if you get chances to spar/train with Michael, take it.
As for knowing the stats gap between no corruption and full corruption...
DrakoGhoul ? You got the number, buddy ?