Great work. Thanks for sharing the details.
Yes, great work but if realism is what you’re going for, the dirt fails. To get that kind of dirt on a new shoe, it would have to be contained in something, splatter on the shoe and then dry instantly when it hits.
To get more realistic dirt, maybe it would be better to cover the whole thing with a general dirt color (brown as you have there) and then start removing it in texture paint. Could use as hard brush as you like, or softer with lower opacity to leave smudges. Could even do multiple versions and layer them afterwards. Then add color variation and textures to those.
You are welcome.
JA12, thanks for advice, but I don’t seek 100-% accuracy. Actually it is a secondary asset in my project. To get the idea where it’ll be, you can check my main WIP here (in Russian).
As for now, here are the recent renders (I’ve modified the shoe slightly):
Ah ok. That explains your interesting choice of different splatter textures. I wasn’t sure what was going on there so I included a conditional when I replied 
Again good work and nice of you to include the steps you’re taking. I’m sure these will help many who know to search for texturing related threads.