Anything works, and Shadowpriests are probably the most viable of the "hybrids".
You have to realize, the raids in vanilla are not that hard, and most guilds these days don't care what you actually bring to the table anymore. These were serious issues back in vanilla, and often times SPriests/Feral Druids/Enhancement Shamans would be left out of Raids due to being offspec, but it matters far less here in 2015. ( And will matter less and less as the months go on. SPriests become much better with 12 debuff limit, Druids here are blessed with an odd bug that lets their enchantments get counted by the Feral buffs, making them raidworthy, and well, Shamans have a lot of problems if they don't go Resto, even in TBC, so bummer for them I guess. )
And a lot of that has to do with how we've changed as a game playing audience. Half the raid isn't on dialup anymore, more people understand the in's and out's of their class, we have damage/aggro meters to better find the people who aren't operating up to par, etc etc. The people who play on a PServer to 60 are generally the "hardcore" audience, so they tend to be better overall when it comes to playing their class properly, especially since there is zero guesswork these days, as it's 10 years later.
A few guilds I play with started doing Ony/MC with just 20-30 man full raids, and they've had zero problems. The content isn't going to care if you go in as an Enhancement Shaman or a Resto Shaman, and as long as the guild has enough healers, they likely won't give a shit if you are cool/can make your case.
With that said, the biggest gain of a SPriest isn't just their damage, it's their utility. Enhancement Shamans/Feral Druids cannot heal effectively, and become a liability on fights like Rags, where they get manabombed/kill all the other melee. If the raid starts going sideways though, an SPriest can drop Shadow Form and quickly take the pressure off the other Priests. Your healing isn't actually worse, you just lack a few minor things other Priests get that give them more longevity over a full dungeon.
If you are willing to, and can take on the roll of a healer when you need to without problems, you'll be fine.