Reforging for Demo for challenge modes:
The more ticks of shadowflame the better, so reforge so it has just gained an extra tick, and be about 6.4 secs duration. This make it easier to catch the last or 2nd last tick with your 2nd HoG, and not as large a %age of the possible ticks. So if your group has 5% spell haste, reforge down to as low as 1330 haste if you can. If not, then aim for 3521 haste. https://docs.google.com/spreadsheet/lv?key=0AsDL8HE-bGDVdFFVWlIzbzQ5RFJJZGFyQ25xZmFYa0E Keep in mind you need to do this in a challenge mode, or at least check your results inside a challenge mode with your gear scaled down. Your hit / exp ratings will be preserved, but not your other ratings. I went mostly for mastery, but also some crit.
Challenge trash is 92, challenge bosses are 93, so it's designed to let you use your raid gear and not be wasting a lot of hit rating. I like to run at 12% hit, which is hit cap for challenge trash. Being 3% under is a dps loss on bosses, but with so little stat budget to play with in 463-scaled gear, I didn't want to waste 1020 rating for most of the instance (3 * 340 hit per %).
As far as gear choice, you're just looking for well itemized gear with as many sockets as possible. Sockets cost 80 primary and 80 secondary stats out of the stat budget of an item, but can be filled with 80 primary and 160 secondary stats, and you get a socket bonus. If you have multiple good secondaries or one of the stats is hit, pieces with nearly equal secondaries will have more total stats than a piece with say 500 haste and 200 mastery.
Due to hit rating being preserved, if you have the normal and the raidfinder of the same piece, you might find that swapping one for the other will make your hit work out just where you want it. This applies with reforging to hit, not just pieces with hit natively. Tweaking that last ounce of stats out of your gear isn't nearly as big a deal as playing well, so if you don't have all day or don't like math, then just make your haste and hit caps and try to use as many pieces with lots of sockets as you can.
Sha-touched weapons are usable in challenge modes, but the 500 int gem in the sha-touched socket doesn't work. The prismatic socket you can add with stage 2 of the legendary chain DOES work. There are some non-sha-touched raid weapons that have regular sockets, so one of those is a good option, too. Jade Spirit is still expensive, so you probably want your weapon to be one piece you share between your pve raid set and your challenge set. If you have a racial that gives an exp bonus with a certain kind of weapon, 340 spell hit is nothing to sneeze at. So if not your current raid weapon, maybe a lower ilvl one that you replaced, but that does have jade spirit and is the right type.
Anything that's 463 or higher is fair game for challenge modes. This includes JP-upgraded 458 JP gear (most doesn't have sockets, so only a good option for slots where socketed gear isn't available). Honor-upgraded pvp gear works well too, and gives a choice of itemization for most slots. And the stats are generally even between the two secondaries, so you're getting the most total secondary stats for your stat budget. Some pieces even have int socket bonuses, rather than useless resil or pvp power. 466-JP-upgraded Zen Alchemist Stone
is a very nice trinket, although I actually use the darkmoon + elegon trinkets. Elegon has a shorter ICD and 20 sec proc instead of 15, so it can proc at the start of many trash packs, further buffing my 2-stack shadowflame with meta dmg bonus. Obviously proc trinkets are great for challenge modes since you have downtime between packs. With so many caster mobs, it's not effective to walk one group into the next.
Remember that you want to optimize for cleave / AoE with a few seconds downtime between bursts. This is why so much of my damage comes from shadowflame, wrathstorm, and stuff like that, and why I gear for so little haste. Haste is not bad for single-target demo, but it's pretty weak compared to mastery and crit. Esp. when you come out of trash pulls with 10 stacks of molten core. I assume that crit boosts the average damage of soulfire more than it does for shadowbolt, but I don't actually know if the gaining damage from crit rating is a better scale than just the chance for sbolt to crit for 2.06 times the normal damage. (Or is it 2.03, I forget when the meta-gem bonus applies)
Other classes will find similar difference between the same; e.g. a frost DK will want to lean towards mastery not haste, for example. (and obviously dual wield, so you can spend your runes on howling blast and consume KM with frost strikes)