I hate BRD.
I mean... I REALLY hate it.... I HATE it.
The powerless feeling I get when I dabbled in it.... uhg... words can not express how much I hated every minute of it.

That being said...
I think that this is something to be considered... Though I think you might be a bit confused with the REM update. They're just changing the base dmg and adding some of that "fake" skill levels to the 90-99 versions (iirc, it might be only 95-99).
So as to the update in things like stats and abilities... I don't believe they've said anything about changing them.

But having done a quick look at the "afterglow" Gjallarhorn.....
There is no "afterglow".
Aegis has an afterglow effect... though I am not sure how it triggers... the fact that it has one and Gjallarhorn doesn't...
But then again... I am unsure of how to trigger one for an instrument and also of what one could be without unhinging things.
BUT... all things considered... seeing as how there IS a 99-2 Gjallarhorn... Why not make the instrument's "afterglow" a spell?
It would give reason to make the upgrade... and could be implemented in the same update that the REM overhaul happens in. (which given the UI going up on the Test Server... I'm expecting it to happen sometime soon-ish... December at the latest if the UI overhaul -which was consuming a lot of dev resources- goes golden in the september update.)

And as to being a thankless role... well... yeah... it really is... (which probably compounds the neutered feeling I had when playing it) But then whms will be overlooked too... and whm and brd are probably two of the most vital jobs in a party... not because they will save the day... but because without them (or with a bad one of them) there is a severe noticeable difference in... well... everything.
And dear god do I thank every player who plays a BRD..... because that means I never have to.