I used a 90 ret pally a few months ago, with some low ilvl pieces off the AH. Was under the impression once I was starting with wotlk dungeons that i had to equip my good geared prot spec, but that turned out to be totally different. As prot it takes ages to kill large packs, and as ret you just breeze through them. Even through the first cata dungeons.

If you have a heroic raid geared ret like you say, you wont take a scratch. Can solo everything up until MoP dungeons with great speed.