Oh, I'm not saying BM isn't capable of doing the damage. Actually, we've toyed with the fact of going BM. But overall damage is also quite important, and Marksmanship is just heads and shoulders better if you take every facet and add it all up.
Overall, I felt my damage could have been better last night, and still, I did what would be considered "great" dps when it comes to other Hunters. I think I could do close to MM numbers on the Tendons if I went BM (and probably 100-300k avg less if Survival) but when add up the other little things, I highly prefer MM to BM.
Nice damage, by the way. You're probably very close to doing the maximum your gear would allow. I would like to add though that the difference in gear isn't going to increase DPS in the 200-400k range per tendon phase. You have a set period of time to do the damage, 23 seconds, and the biggest difference in DPS will be either Crits or no Crits. If I used your gear and had the same luck as with my gear, I'd expect maybe 50-100k gain switching to my gear, and that's on the high side. One of our other Hunter's that has been in on attempts with very similar gear to you is pulling numbers quite close to Wyvern and myself.
You did mention you don't know if I had bad luck or good luck, so here's overall tendon damage for the night. When only Legendaries + Rogues are beating me, I'd say we're in a pretty good place