My personal feeling is that conventional knowledge, common sense, and the sim are wrong, but I don't have any proof. Do my work for me and prove me right and everyone else wrong please.
I have this idea that Fire Nova may be our best glyph, single target included. My theory is that we are not as GCD strained as we think(or as the sim predicts?), and by glyphing FN we can cast roughly 0.06-0.1 additional spells per second. In practice this means roughly 1 extra FN every 10-16 seconds which is ~160-220dps
Looking at logs from Deathbringer Saurfang counting the total casts of, LB, ES, FN, SS, FS, LL, CL, and the assumptions that people are keeping up totems, ls, lust, wolves as normal.
3 Shamans, un-glyphed .567spells/second - ex. Rouncer
2 Shamans, 3 logs FN-glyphed .643spells/second ex. Kresh1, Kresh2
Of course there could be other explanations: The sim could be correct in that we are GCD capped, and factors like haste, latency, autohotkey, and correctly prioritizing spells prevent players from reaching the same spell/second rate.
The poster makes an honest effort to bring evidence to bear in support of his argument and raises a reasonable point. Although the point he raises has already been addressed, we don't wish to punish posters who earnestly and constructively (even if redundantly) contribute to discussion.