I realize that destro is supposed to gem Int/hit in blue sockets, and demo is supposed to gem haste/hit. And destro should reforge to mastery while demo reforges to haste. I will primarily be raiding as destro. If I gem and reforge destro, will I be okay for demo?
You would be OK, but just OK. DPS would be gimped to say the least, as your haste would not be as high as it should be. You would be good with Destro/Affliction, but Demo is not extremely viable in Destro gemmed/reforged gear. Hope it helps, happy warlocking!
If you're doing what I'm doing, and going Demo on the add heavy fights, you'll be fine. The added dps from improved hellfire + viable shadowflame use + demon immo + felstorm makes Gunship, trash, Dreamwalker, etc, far more fun, even while geared for Destro.
you still need the same amount of haste for both, that doesnt change. ie aim for 1406 haste and dont go over thresholds more than you must. mastery is pretty mediocre for demo but its FAR from terrible. it wont "gimp" your dps, the actual number differences are minor at best.