Just to let you know, I have managed to upgrade the Sagepay server plugin to work with Nopcommerce 3.80.
Drop me an email if you want a copy of the code, although the changes were minor and I have reworked the configuration page to come in-line with the new look and feel of the new Nopcommerce Administration UI (probably not perfect but better than what was presented on the first port of the code).
It is working on the areas I have tested it for Payment, Refund and Partial Refund under the PAYMENT option which my clients use and no issues reported.
Regards
Matt