In the context of coder, the userspace TCP overhead should be negligible. Based on
https://gvisor.dev/docs/architecture_guide/performance/ and assuming runc is mostly just using the regular kernel networking stack (I think it does, since it mostly just does syscall filtering?) it should be at most a 30% direct TCP performance hit. But in a real application you typically only spend a negligible amount of total time in the TCP stack - the client code, total e2e latency, and server code corresponding to a particular packet will take much more time.
You'll note their node/ruby benchmarks showed a substantially bigger performance hit. That's because the other gvisor sandboxing functionality (general syscall + file I/O) has more of an impact on performance, but also because these are network-processing bound applications (rare) that were still reaching high QPS in absolute terms for their perspective runtimes (do you know many real-world node apps doing 350qps-800qps per instance?).
Because coder is not likely to be bottlenecked by CPU availability for networking, the resource overhead should be inconsequential, and what's really important is the impact on user latency. But that's something likely on the order of 1ms for a roundtrip that is already spending probably 30-50ms at best in transit between client and server (given that coder's server would be running in a datacenter with clients at home or the office), plus the actual application logic overhead which is at best 10ms. And that's very similar to a lot of gvisor netstack use cases which is why it's not as big of a deal as you think it is.
TLDR: For the stuff you'd actually care about (roundtrip latency) in the coder usecase the perf hit of using gvisor netstack should be like 2% at most, and most likely much less. Either way it's small enough to be imperceivable to the actual human using the client.