As of 8905, Misery now supplies its 3% spell hit as well as a 15% spell power coefficient increase on Mind Flay, Mind Sear, and Mind Blast. It will be interesting to see how this plays out, and just how much a difference that makes. The original version of Misery was 5% spell damage across the entire raid, including your personal damage. Mind Blast and Mind Flay are definitely huge sources of a Shadow Priest's damage, but it's less than half of the spells used in our damage 'rotation.'
Will 15% more damage on those spells make up for the loss of 5% damage on all spells, as far as your personal damage is concerned? It's certainly possible, probably even likely. While the damage from Shadow Word: Pain and Shadow Word: Death (especially Pain) shouldn't be underestimated, Blast and Flay are definitely the major players here. Plus we have news that Mind Flay's coefficient is going up and gaining the ability to crit soon, so an extra 15% on that spell will scale very, very well.
Now that Naxxramas has opened up on the beta realms, we should start seeing some numbers and math soon. However, the new Mind Flay isn't implemented yet, so how this scale may completely change when that happens. At the very least, the bare minimum, this is an improvement over what they did to it originally. How much of an improvement remains to be seen.
Edit: Fixed to reflect the talent's wording.