What made you go into Utility over Offense?

Personally I think it lacks magic pen which looks like a rune issue. Maybe Insight marks instead of AP marks would help.

I like the idea of CDR for faster spams but you still have overheat capping your spells. I'm unsure if this is a good/bad thing, will definitely test it though :)

Have you tried maxing out harpoons as a priority over fire? It keeps the lane zoned better and makes Rumble a better ganker immediately imo. Allows for better jukes and stronger slows faster.