I have used the non STM version as well, while it is solid and you can get good results until 200mm, it is nowhere near any L lense I have used. Honestly mate, I get your problem, but I would struggle having to use it again. Knowing that you are quite picky with your shots as well, not sure if it's made for you.

For my previous "non-L" Standards it was okay though within its limitations.
I don't see why the STM should make the image quality much better. But again until ~ 200mm you should get decent results.
Generally I always thought and still think that it does offer great value for money.