I have use http2 and http1 with domain shardind extensively and the advantage of http2 multiplexing cannot be denied. TCP connection overhead is negligible (other than the tsl negotiation maybe) and is not even really a factor.
TCP congestion window scaling is not negligible, especially not in high-latency environments. The initcwnd is usually 14,6kb(10x MSS of 1460 byte in common DSL setups), meaning a server can only send those 14,6 kb until the connection is stalled and an ACK is received (=1x roundtrip). Google realized this and tried to get a kernel patch into Linux where the browser (or any userspace program) can manipulate the initcwnd. The patch got rejected, and thats basically why they came up with QUIC: To be able to build on UDP and implement congestion control and other stuff TCP usually does (ordering, checksumming etc) in userspace.
