Started out with 4# Fireline Crystal braid. At present I'm using Diawa J Braid, 4x, 6 pound test. I only spool on about 100-120 feet of line. Where I fish, there's no need for more and less line means a lighter spool, the name of the game in BFS.
I also have an Abu 2500C set up for BFS-ish work. It has 10# Diawa J-Braid, 8x on it. It can cast down to 1/16 of an ounce stuff pretty well. I've even tried 1/32 heads with a bit heavier plastic lure on and was successful for short casts, 30-40 feet. I have a lightweight spool I will try at some point, just to see what difference it makes.