RCU doesn't imply shared-everything, just shared-a-few-things-under-very-carefully-controlled-conditions :-). There's nothing stopping them from adding an experts-only, carefully-bounded escape hatch in the standard library. Or from using RCU under the covers to implement something with those semantics -- Rust doesn't literally use 'fork', language runtimes can be very tricky about how they implement things. OTOH you can't add memory isolation in a library :-).
But yeah, it's possible that in some situations, code in Rust will be slower than the best possible concurrent implementation that exploits details of the CPU's cache coherency model etc. For me this is a totally acceptable trade-off, but again YMMV.
(Anyway, it doesn't look like Go has any primitive memory barrier operations, so your only safe concurrency options there are mutexes and channel sends. No RCU.)