I just went and shot Wilwood an email asking basically to confirm those assumptions. Will post their response if/when it comes.
Thankfully, a larger bore master cylinder is the one thing that's easy to find among this mess of part numbers.
Thanks for the lead on ANG, I saw Rimmer got them in stock as well, but at atrocious prices.
May go the rebuild route instead, as the 14s are also a bit heavier. No sense in adding mass (and unsprung!) to something that's already overkill. My Europa has a back exit on the calipers, which I haven't seen on a Girling before. Unsure if there's a reason for that? Seems like it would make bleeding maybe easier? Haven't dug into the front suspension yet.
Just need a spare set of pistons sitting on the shelf, in case they're ever gone-gone.
Hopefully those thoughts and Willwood's response may be helpful to someone in the future regardless.