I've written some high req/sec projects in Rust, but nothing that would be apples-to-apples to that particular Go project.

The memory access patterns are straight-forward enough that the remaining GC could definitely go away. If I had to make a wild, from the hip, and unsubstantiated guess, maybe a 2x to 5x performance multiple. That would include a comparable amount of time spent optimizing and would depend greatly on the quality of the Kafka crates for Rust (which I've never used).