Preclearance would be a pretty big deal and I could definitely see an uptick of passengers using Edinburgh to travel across the pond. With the A321XLR coming pretty soon with orders by United and AA, I'd like to think that more year round services would be viable.
It's worth noting that in the depths of winter, Shannon only serves one transatlantic passenger service which CBP remains open for. Appreciate that wasn't the case when it opened and it's been established for a lot longer they still seem to make a case for it.
I keep banging on though that the check in area needs to be vastly expanded though!