Sepherosa Ziehau has added a sort of queuing to altq, where TCP ACKs get higher priority.  You may have seen this in any number of pf configurations, where returning data is given its own queue to keep high-volume transfers from slowing themselves down because the acknowledgements can’t get back to the sender.  His commit has statistics on the performance improvement.  He also added a ‘netrate‘ tool for calculating results from using netperf.