I suppose the makers of Nim named that flag for a reason - giving up memory safety by disabling bounds/overflow checks should be never be the default for networked software in a production setting, so benchmarking in that mode would paint an unrealistic picture.

What you want are comparable compiler flags across languages, say "optimized for performance, yet retaining safety" and "go as fast as possible and disable all brakes". Which, to be fair, is the default for C anyway, but this is not a desirable default. I'm sure you can similarly game Rust bechmarks by using "unsafe".