this is a really tough issue, sounds like youve done your homework and still no answer in sight. assuming this is a guild run you may just have to explain to these dps'ers that your nuking the shit out of the meter, and can simply cannot keep up in TPS, watch your aggro. since your on hardmode stuff im going to assume that they will understand and hopefully this might help out. (this is just a temporary fix i realize)
does the other tank have an equally hard time keeping up?
you mention lag, whats it typically at? because that would deff set your total TPS back if its bad,
edit: perhaps some of the DPS (rogue and hunter) may have to add tricks or MD into their normal rotation to help you keep an upper edge on this killer dps you have behind ya?
i can understand your quest to be the top of your game, but this may be what it takes as part of maxing out your role and using all utilitys available to you