1) Destro. Apparently Demo used to be viable but not anymore due to rebalancing early in MoP. But you could probably get away with the other specs if you really wanted to and the group was okay with it, it's just that Destro's AoE is hard to say no to and looks so sexy
2) GoSac, simply because not having a pet means no pets accidentally pulling during an invis run or running down the ramp to Gekkan etc. If you really want to the other Grimoires work just fine, I've seen people using pets in plenty of runs. GoSac just means one less thing to manage.
3) Legendary scales down like anything else, proc doesn't work, however it has nice sockets. Not exactly sure how the scaling works on the three stats (I've noticed that gear with only 1 secondary stat doesn't scale properly) but safe to say the cloak is a strong choice for CMs if not BiS. Just remember to change out your helm for a regular meta gem.
Also, if possible get gear with as many sockets as possible, ToT weapons are good because they have one socket plus the one from the Eye of the Black Prince (only legendary reward that works)
Most important things as a lock: always have RoF up for AoE pulls (90% of CMs), use Havoc with CB/Shadowburn where you can, and of course Shadowfury is the absolute shit (for best results coordinate a stun rotation with your party). Don't be afraid to drop an Infernal on some of the harder trash packs for the AoE stun as much as the damage. Also, your pet (sacced or otherwise) should be a Felhunter for the interrupt most of the time. Interrupts are important! Oh and Soul Leech is OP as fuck. Can eat AoE that lesser mortals need CDs for :P
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Only down side of that strat is people sometimes die after boss is dead because healer forgets about dispels :P