As a protocol tcp should be able to utilize a long fat pipe with a large enough receive window. You might want to check what window scaling factor is used and look for a tunable. I accept that some implementations may have limits beyond the protocol level. And even low levels of packet loss can severely affect throughput of a single stream.
A bigger reason you want multiple streams is because most network providers use a stream identifier like the 5-tuple hash to spread traffic, and support single-stream bandwidth much lower than whatever aggregate they may advertise.
Yeah, that's the issue. I didn't have root permissions on either side. Moreover, a transfer tool should just work without requiring its users to have expert knowledge like this.
In this case, I checked the roundtrip ping time and multiplied it by the buffer size, and it agreed with the speeds I was seeing within ~5%, so it was not an issue with throttling. Actually, if I were a network provider interested in doing this, I would throttle on the 2-tuple as well.
A bigger reason you want multiple streams is because most network providers use a stream identifier like the 5-tuple hash to spread traffic, and support single-stream bandwidth much lower than whatever aggregate they may advertise.