Most hardware (at least, that I've used) doesn't care where your release knob is set if "Auto" is engaged. Although, come to think of it, most hardware (that I've used) uses the release knob to select "Auto" mode, so you can't really get around that anyway.
I've come across some plugins that seem to pay attention to where the release setting is, even with Auto mode engaged. I think the Waves Rcomp does this (??). Not 100% sure, though; I don't use a lot of plugin compression.
It shouldn't be too hard to figure out on a piece-by-piece basis, though, if you're wondering about specific gear. Pump something bottom-heavy and transient-heavy through, like a kick or a short bass guitar note, and play around. It oughta be relatively easy to hear the release times, or, at the very least, watch the GR meter for changes in behavior.
As for positive values on thresholds, once upon a time, in a dark and distant land before the age of the DAW, 0dB was not the absolute maximum level a signal could reach before clipping. 0dB would indicate nominal operating levels, and the positive values above zero would be your headroom. So, a console like an SSL, which has a good chunk of headroom (what, like +26dB or so?) will need a compressor whose threshold can be set accordingly.