in my gear barrage does more damage even single target then 2 GT... the only reason i use GT is because of criting if on glaive crits it is often enough damage to win out, however without critting it always loses. With BM you really don't lose the GCDs people make it out to. for me i oftem gain time as with my haste if RF or FF is up its a sub 1 second cast.. hell at the start with RF and BL going i have had it hit 1.1...
Thinking about it i might run with it next week full time just for S&Gs...
the other thing to remember is glaives is a "skill shot" if the target moves too much both glaives won't always hit, that not something that can happen with barrrage.