Rails proper, yes. Small rails app are generally drop-in compatible, but sizeable applications are likely to run in a few compatibility issues as most gems aren't tested against TruffleRuby.
> I wonder how much TruffleRuby would improve the performance and memory footprint.
The generally speaking Truffle is much faster at "peak" performance, but take very long to get there which makes it challenging to deploy.
It also uses way more memory, but it's partially offset by the fact that it doesn't have a GVL, so you get parallel execution with threads.
Ruby atm is working towards implementing true parallell execution with Ractors for example, and now with YJIT, the performance might increase some more.
Especially with native images, I wonder how that would turn out.