Toke,
great piece of work.
UDP tests show that the scheduler is working quite well.
TCP tests: no news that ACK streams are a problem.
This is another well known issue in wifi.
Every data packet is ACKed already at L2 and L4
sends an ACK that is non contention free.
There is an NSDI paper in 2014 that proposes to use the L2 ACK to carry the L4 ACK.
It works well but purists might hate that approach.
BTW the gains you show are huge. If you add multiple stations uniformly distributed around the AP you'll see tremendous gains.
The delay gain is also huge and poorly explored so far.