At the moment, you can only scale one track's waveform at a time, or all at once, via a key binding.
 
Sometimes, I want to scale a bunch of tracks by the same amount, but quick grouping doesn't work on this feature. Would  be really handy.

Sounds like a great idea. +1

+1 not a feature i would personally use a lot but great idea. It gets back to the premise of universality/standardization of workflow. Currently there are still a lot of inconsistencies in what works where in X3 that hopefully get reworked and streamlined in X4+
Quick groups should ideally work for any edit function for which a single selection can be made.