Good question.
I believe that Classic Mode FTW would make the enemies & bosses have Expert-level stats, and the bosses would have the unique FTW changes but without the expert mode patterns.
If I had to guess, it would probably be a similar level of challenge to regular Master Mode considering the world itself is a lot more dangerous with bombs in containers and lava everywhere. It is difficult to say for sure with the bosses not using their Expert Mode patterns. It is possible they're still in FTW seed worlds regardless, but I'm not sure about that.