My 5x Priest composition had a much easier time leveling that any other caster composition.
The only set of five toons who leveled quicker were 5x Ferals.
Not that I've done every composition.
And I've only taken them to 70th (or somewhere barely beyond that point).

5x Shadow will have some decent potential, because of all the passive healing.
I'd personally go 5x Disc, or a combination of Holy/Disc.
Even if you're not as deadly or quick to kill others, five on one will take out any equal geared opponent.
And if you stick with the masses in AV/IoC, healing makes a far greater impact on the battlefield than does damage.
Chances are Shadow will do better in the smaller scale BG's and a healer spec in the larger scale (that is entirely supposition on my part, but makes sense).

Nothing precludes having dual-spec.

Also, consider playing them in the 70-74 bracket, 70 if you want to arena.
Or possibly in the 80-84 bracket, as an 84.
Stop experience at the target level.
Both are very large twink brackets, and at times Blizzard has stopped experience players in the same pool as regular players if there aren't enough players with experience turned off.