Have any of you used FireLine for backing?
Wouldn’t matter what I used, I never get to see it.
no but if i was going to use a gsp like that i would just buy the stuff made for fly fishing 1. better deal2. theres more of it than a single spool of fireline
but im sure you could use it if you wanted to! 
WHY NOT???
Never seem to get a fish big enough to take me down to it!
You are better off buying a high quality Spectra product (if you are looking for low diameter high strength) as it will last longer and hold up better. Both PowerPro and Western Filament (Tuf-Line) make good lines that are great for backing.
MICRONITE “MICRO-DIAMETER” BACKING, good stuff. 
Gringo, Hopefully you WILL see your backing in November!
All the best.
Mike.
I use Fireline I had from my hard tackle days. It works just fine and with a smaller diameter, I can get more on.