This turned out to be a driver issue, related to having an incorrect version of Bootcamp installed. (This was a result of having moved my Win7 partition from the old machine via Winclone.
Uninstalling Bootcamp (in order to get the new version installed) was a bit of a pain. Whenever I tried to remove the Bootcamp Services app from the "Uninstall a Program" dialog in Win7, I received a message
This version of Bootcamp is not intended for this computer model.
Finally, I used the Microsoft Uninstall Repair Tool, which modified something that let me uninstall the old version of the Bootcamp utilities.
I then ran the new
Bootcamp\setup.exe that Bootcamp Assistant downloaded for this machine. Installation proceeded without a hitch, though it took ~10 minutes to complete. I then re-ran the "Windows Experience Index" assessment and got a respectable 7.2 for the SSD:
Surprisingly, still lower than the 7.9 I got on the mid-2012 15" Retina with 768GB SSD.
The WinSat command-line tool provided more insight:
with a read speed of 1181.8 MB/sec, and a write speed of 1019.66 MB/sec. This is roughly equivalent with what BlackMagic reports under OSX.
(Still, I'm wondering if Windows gives speeds > 1GB/sec a 7.2 of 7.9, what on earth do they consider to be a 7.9?)