This enhancement is available for 5.5 in patch NSTFDD9A or a successor.
One of its successors (I don't recall which one) went GR earlier this week.
It is also available 6.0 in a similar patch, but don't ask me which one.
Be forewarned, however, that if you use 'setsockopt' to alter the size of
the TCP receive window, you may run into problems. (Very few people
explicitly set the size of the TCP receive window; only one that I know
of.) There is an even newer patch in very early testing should you run
into that problem.