[robocat]

The expression “shit hot” is a superlatively good thing. I don’t ever recall hearing “hot shit”, although perhaps it is a US term or a modern usage. Urban dictionary defines both.

[lamontcg]

"Hot shit" is a US term, and it is old and was around in the 80s at least.

[reaperducer]

Longer than that. I saw it in a book from the 1950's.

I expect it's a variation of "hot stuff" which I've heard in songs from the 1930's.