highvoltage12vSame here one of my Mac Pros still have an EFI gtx970 in it that was NVMe flashed on 089 and High Sierra. I just extracted recovery from one of other Mac Pros, and ran it on the High Sierra install. Retained NVMe support while gaining the the newest firmware and still on High Sierra.
Thanks for your feedback, I'm in exactly the same setup as you. High Sierra bootrom MP51.0089.B00 & have been booting from a Samsung M.2 NVMe 970 EVO. Your post was exactly what I wanted to hear.
Did you notice any obvious improvements booting into & using High Sierra 10.13.6 with the 140.0.0.0.0 bootrom ?
I won't be installing Mojave until it is finalised but am looking forward to the improvements we've been hearing about.
tsialex very kindly created the 140.0.0.0.0 bootrom for my 4,1>5,1 tonight .. I'm cloning my boot 970 EVO now to a spinner HDD in case anything untoward should occur.
I'll post my findings after the upgrade.
Hopefully the next bootrom upgrade will allow USB 3.0 booting - fibers ( & toes ) crossed.