Prot pally would be worst solo grinding char to use. Unless you multi pulls. Prot Pally has the worst DPS among all tanking classes/spec. But they scale relatively well with Vengeance, you need to get hit a ton first, before you see your numbers go up.
Your stats and reforging is a big doubt too. A prot pally would do well DPS wise at around 40% haste, with active mitigations coming from your skills, and HASTE reducing CDs on your mitigation like Avenger Shield, SoTR, HoTR, Crusader etc.
And you are right with one thing. Prot tank DPS is shit. Unless you get hit a ton, else you're shit.
Just to add:
My prot pally is at ilvl 545 and doing about 80k dps if I multi pull Ordon mobs like 3 of em, takes like a min to kill.
My ret pally at 520 doing about 150k dps if I multipull also, takes like 30 sec or less.
The thing with this CR is you could go RET, and worry less about dying. Configure your settings properly and use SoInsight for longevity and sustained boost to dps.