It was already written in a very performant language so no significant performance improvements should be expected.

The benefit of a rust rewrite is memory safety improvements, but currently they've just rewritten zig to unsafe rust so they don't have that either yet

The blog claims rust chosen mainly to address memory issues, which rust is a better language for. So, success would simply be less new memory errors / easier to patch old ones.

I don't think their motivation was primarily user-visible stuff. They backed themselves into a corner by forking Zig, and also were fed up with fixing memory errors.

You wouldn't notice either of those if you were a user, unless you happened to hit one of those bugs.