Healing as a priest with DPS gear

I'm almost done leveling my priest and plan on going shadow in the long run. I've already farmed a bunch of materials for gear (wolven leggings, deathfrost boots, Merlin's robe and the wrist from ToC) so I'll be pretty set regarding some starting items.

My question is; can I get dual spec and heal heroic dungeons with said gear? I plan on shadow being my primary spec so I obviously want to gear as an rdps first but it'd be quicker to just que up as a healer and get a secondary spec right away. I've never healed before but I know spirit and haste are the primary stats (at least for holy) and the gear that I'll be wearing is devoid of both stats. Is it possible to efficiently heal heroic dungeons as a holy/disc priest wearing shadow gear or am I going to run in a lot of mana issues?

A similar question was raised not long ago on this thread: http://forum.warmane.com/showthread.php?t=422414
tldr: it is possible to heal in dps gear, although you will never be as efficient as a healer with proper healing gear. I think the biggest difference comes with the tier-sets, trinkets and items with hit stats (you don't need that on a healer, and you are missing out on other useful stats).
My advice would be to start gearing as your main, and grab some off-set items on your way for your offspec.

You are completely fine healing heroics in DPS gear. Just make sure you bring drinks because the biggest issue you'll run into is mana.