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.