if VIA will be equipping Corridor trains with ETCS gear, wouldn’t that get some of the expense out of the way for deploying the same tech on their own trackage?
Yes but only a small proportion of the total expense of adding a new signalling system. However, if their trains are already ETCS-equipped then that would certainly encourage them to select ETCS if they're looking for a new system, not just to avoid of the cost of maintaining two sets of onboard equipment, but also to reduce training requirements and potentially reduce the number of signal system transitions along the route.
The question is what Via would gain from upgrading to ETCS. Safety (automatic train protection) is the most obvious one, but that doesn't seem to be a pressing priority for Via or TC at the moment, probably because there haven't been many Via train crashes recently. Unlike GO, it's unlikely that Via would have any routes entirely within ETCS territory, so they wouldn't get the cost savings of reducing from two engineers per train to one.
Capacity is not an issue for Via, since even with HFR we'd likey only be looking at a handful of trains per hour.
Speed is a potential advantage, since ETCS offers cab signalling and ATP. I'm not sure what TC's rule is about max speed while depending on line-side signals, but many countries require cab signalling and ATP for speeds over 160 km/h.
Given the above, maybe Via would be interested in ETCS L1 on Via-owned lines, since it gives them the speed and safety improvements while being simpler than L2 and potentially cheaper for installations such as Via where block lengths are quite long.