I don't have any premium tanks and I'd say I average at least 1 game a night where I'm top tier and several where I'm top 5. It happens about as often as I'm bottom or bottom 5. My M7(t5) and E8(t6) are maxed out and I have a lot of exp in them.
My t-20(t7) isn't yet, but it's pretty close.
My theory is that the premium tanks are on top more because they get played more.
I've found the time of day seems to make a difference. Earlier in the day I think there are more lower tier tanks playing so I get stuck at the top. As the night goes on the heavy hitters come out to play and I get the crap kicked out of me.
The Matchmaker has to work with what is available since there are no bots.
On a slightly unrelated note...
I was just looking around in the forum and discovered that if the MM can't find a match for you in 5 min, it gives up and just throws what's available into a match, resulting in small unbalanced matches. i.e. a 4 on 4 battle with heavies on one side and lights on the other.