Thanks for the feedback! I did end-up getting a used case, a 55 liter Monokey (forgot which model exactly) with lights kit. It is huge, but will hold my helmet and jacket with some room to spare. Still haven't installed the adapter, so have not actually ridden with it yet.
I don't know if you've picked a case yet, but I thought to share my short experience regarding the model Zero sells (I received/installed it a little over a week ago) with you, or anyone reading this who wants input. I think it looks good (although it does border on looking silly when I'm off the bike, like most rear boxes, because of how high it's set), it matches the bike, and I can't feel it at all while riding. The one downside would be that a full helmet does take up most of the space. I noticed in the instructions/pamphlet that the suggested top speed was something like 70-75mph but I end up going faster than that with it on and don't notice it shaking or moving any more than at slightly lower speeds. It only moves/vibrates very slightly. I can take a picture of it but you can just find a bunch of pictures by google image searching "ZERO TOP BOX BY GIVI".
The huge case you're describing is a Givi Maxia (gen 3 or 4 usually). I've used one for 5 years and Terry is using one on his blue 2015 SR.
The OEM case is a Monolock (different plate) E340 and is quite small.
In principle there is a concern about the weight at the end of the top rack as a lever on riding dynamics and in strong gusts at high speed but the warnings are more cautionary. Mount it solidly and don't put bricks in it and you should be fine.
That said, an Elcon or Supercharger in the top case kind of makes me nervous personally for that reason. Reinforcing the top rack would be good for Zero or for us.