First thought is that players that use excuses to force their raid to do things differently are the ones that need to alter what they're doing. As a PT tank, I can understand the idea of a delay, since there is a visual and audio delay. Without doing specific testing, I can't agree one way or the other, but I'd bring up the fact that the debuff isn't necessarily the only factor. At the core, it's when the mob turns that really matters, and after leveling a sin recently, it certainly feels like mobs are quicker to turn for a sin. That obviously is probably not true, but a different perspective.
It sounds like you might have touched on a different topic here though, which is the sheer number of encounters that require tanks to manually target mobs that do not yet exist or are "green" (examples: WH jealous male, Operator IX adds, TFB tentacles, Minesweeper Droids, Kephess trandoshan warriors, Nightmare Pilgrim and TFTS). This is obviously an added task for the tank, as he has to get the thing targeted (and not the wrong thing targeted in many cases), taunt, and at the same time build an insane amount of threat to keep up with dpsers who are now opening with 2k+ dps. Where I can, I open with flame thrower so I can get initial aggro, but it certainly becomes a pain in NiM kephess trying to drag warriors out of packs. Nothing more to say about it, except that in those instances you just have to hope for the best.
I guess I'd be interested in knowing the encounter where dps needs to wait. If you're in NiM EC and you're not doing a count down, you should probably start. That allows dps to know when the taunt is going to hit. If they can both pull aggro and also take damage from the boss in the time between when your tank says 1 and GO, you've got some very precise dps