Glad I found this thread.
I've been requesting this for almost 2 years now, since I started beta testing BFD3.
BFD3 has a similar feature in the mixer where by you can click on +/- buttons to show and hide each kit pieces sub channels.
This helps free up space in the mixer a lot and in truth there aren't normally more than 3 or 4 sub channels in each kit piece anyway so imagine how much more useful it would be in sonars mixer/console when an instrument like BFD3 can have 16 to 32 separate channels.
You could even group channels in sonar and hide/reveal those also. They should never be completely hidden (that's what the track manager is for) but should always have a single "master" channel on view.
This would have the +/- button on it to reveal it's subsidiary channels.
Steve