Recklessness, my man.
Shock > Wither > Discharge > Taunt (between GCDs) > Shock > Recklessness > FL > normal rotation > nap
With ubergoober DPS, you can always chuck the AoE taunt as soon as the single target wears, assuming you don't have another need for it in the following 45 seconds (unlikely, on any current boss).
Edit: For the record, besides taunts, FL is the 2nd largest contributor to single target TPS. Shock is #1, FL is #2, Wither is #3.
I usually use one Thrash/Double Strike after the Discharge and a 2nd one if it doesn't proc the shock, it is possible to lose aggro if you wait for shock to be ready by its natural cooldown. That's usually one of the rare spots for Thrash, I also still try to proc Energize for the 2nd charge on Recklessness like a total noob but I reckon it's better used on Wither.
If you can afford it (according to the boss fight) you can start with Force Pull for a base of 5k threat, it's not as much as a procced shock but better than a non proc shock. Anyway, only boss fight where tanks can get into trouble is the TFB on 16 man since it's unfeasible to guard all the dps or have the "best" tank take the best dps like on 8 man. But here you can use the orbs I guess, never tried those on HM, or you can walk 4-5m and taunt.
On the note of TPS which I've tested, I presume I can get decent numbers on my assassin as a dps and at 19 sec in the fight I'm sitting at 2.5k dps, 38 sec it's 2.4k dps. This is with using everything for a burst as soon as the fight starts. While tanking I got 45k threat or so at around 20 seconds last time on the tentacles (10% gain from taunts instead of 30% since I keep staying close). So yeah if a dps pulls around 3k dps from the start and keeps it up for more than 20-25 sec (3 taunt rotations + start of the fight) without ever dropping threat they'll probably pull :-(
P.S. I forgot what I actually wanted to say when quoting you and remembered: Recklessness won't save you, yes it increases the crit chance by 60% of each FL tick but it's only a 51% increase in damage if they proc, and not all of the ticks proc. So overall it's nice to use but if you would've lost aggro without it, you'll still lose it with. Focus on rotation/ augments (12 power on each) first.