You can boost up to 80th.

That is contingent on being able to solo (with your booster), the instance you are boosting in.
I'm currently using an 85th Frost Death Knight (in almost full Conquest 4.3 gear), so pretty good health/resilience and likely low damage compared to a PvE geared Death Knight.
Still, the class has a lot of AoE power (Howling Blast) and wears plate, which helps.

The most difficult instance I am using for boosting is Hall of Lighting.
That is for 78th to 80th.
Not sure if a Priest could solo it, but there's one way to find out.