hmm, not sure if this helps but "excess queueing" can be hard to define.
Do you know the operating systems for the WiFi devices and if tooling can be loaded upon them? iperf clients samples RTT and CWND for linux machines. Iperf 2.0.14 (in development) has a lot of latency related features
Also, if there is control over the AIFS one can set that for the high rates devices such that they always win and the lower rate ones always lose. If that solves things it does suggest WiFi tx queues developing per the TXOP arbitration and air transmission as an issue. Standard cwmin/cwmax isn't as effective though it won't allow high rates to starve low rates devices as AIFS might (depending upon the values)
I use latency to measure the performance and define bounds that way and it's very specific to use cases. IT does require clock sync. My devices have GPS disciplined oscillators which aren't common.
As an aside, the HULL approach of phantom queues looks interesting.
https://people.csail.mit.edu/alizadeh/papers/hull-nsdi12.pdf
Bob