I think that you are spot on with most of what you describe for both tanking and healing as you level through the dungeons. The common factor with both roles is that you can only influence your group to a certain point. You will nearly always have a mix of players, both experienced and new, and your runs will only go as fast as the group as a whole can handle. As a tank, you can compensate for 1 and maybe 2 dps that might be new and/or struggling, especially if you have your heirlooms and know what you are doing. But you can still only proceed as fast as the healer will allow without wiping.
Same thing goes for healing, even if the tank in your group is a real gem, the dps might be some flavor of crazy and you can't heal 4 people all taking damage at the same time and expect the run to finish in record time.
I guess what I'm saying is that you have to allow for the fact that nearly every group you have will have 1 or more new, inexperienced players, and also might contain 1 or more complete jerks that have multiple 80's and just like to run lowbie dungeons to aggravate other people for their own enjoyment.
Personally, I see my tank queue almost instantly while my healer can be almost instant to nearly 10-15 minutes sometimes. I think you have more control over the run as a tank, but again you can only do what you can do with the other 4 players and hope for a run that goes well and fast.
Good luck and let us know how this turns out for you.