... what if you want to strap a stereo compressor across the mix buss and you also instantiate latency inducing plug ins in its chain?
By 'chain', do you mean latency inducing plugins on the same output
channel strip ('mix buss')? Or do you mean latency inducing plugins on an Aux, before the audio reaches the output channel strip?
Either way, recording the signal returning to I/O plugins on output channel strips is very restrictive, and will only work sample accurately if the outboard itself is zero latency, and other strict restrictions are adhered to (see 'Recording an I/O Plugin from an Output' from this post)
. I suggest you create an 'output Aux' that's simply an aux that acts as a 'middle-man' just before the output channel strip. Instead of routing tracks and aux-sends straight to an output, I route them ALL through the 'output Aux' first, which is the only channel strip routed directly to the output channel strip. All other tracks/auxes go through that intermediary aux first. I never route audio direct to an output channel strip on my system, I have an 'output Aux' for each of my output pairs, and put latency inducing plugins on the 'output Aux' instead of the output channel strip. I think this is pretty much answers your question below:
Would I then need to send the output to yet another bus before bouncing?
Yes, as I said above, use an intermediary aux and put the I/O plugin on there and only
route the intermediary aux to the 'mix buss'. I/O plugins on Aux channel strips are just easier to work with.
However, you'll be fine recording the return signal from an I/O plugin on an output channel strip so long as the outboard is fully analogue with zero latency (i.e. the I/O plugin's latency is exactly the same as your usual roundtrip latency). But you also can't have any latency inducing plugins before the output channel strip's I/O plugin, either on an aux (send) or inserted onto the output channel strip before the I/O plugin (but you can insert latency inducing plugins after
the output channel strip's I/O plugin, and latency inducing plugins on audio or instrument tracks
are OK). Basically, the output channel strip's I/O plugin must be the first latency inducing plugin after your audio/instrument tracks.
^ But on the whole, it's just easier to record an I/O plugin's return signal if you put it on an aux. See 'Recording an I/O Plugin from an Aux - Method 2', again from this post.