As it happens I’ve been doing once again some more testing in that regards as well.
One of the problems I’ve found is the video decoder in QGC is much slower than in MP (gstreamer).
So no matter how good your tx is the bottleneck can be down to software and or hardware on the ground-station end as well.
There appears a lot of interest in the SIYI HM 30 which has just recently gained FCC approval in the US.
But have to admit I have not tested this unit as I currently use 3G/4G connectivity instead.