As I said in my original post, I believe this is true up to a point. I agree with you concerning twice the speed limit. If you are going that fast, then yes you are far more dangerous than someone doing 80 or 90 Kph.
Now my only real experience is here in Ontario. Here most drivers on the 400-series highways (similar to interstates) drive between 110 and 120Kph (the official speed limit is 100Kph). In this instance, the drivers going 80 or 90 Kph are more dangerous, from what I've seen. It's even worse if you are driving that slow in the left hand lane (the passing lane). Here is illegal to pass on the right (though people do it and I haven't heard of anyone getting ticketed for it). So the leftmost lane is supposed to be used for passing not driving steadily at 80.
The biggest problem with the way a lot of people try to merge onto a highway is their speed. If they get up to at least 90 before attempting to move over onto the highway, most wouldn't have a problem at that point.
In the city, it's not as big of a deal. Yes it can be annoying to sit behind someone doing 30 (most city streets have a limit of 50). But it's not as dangerous. I tend to drive around 10 over the limit no matter where I am. This is not far from the 5 over that you mention you do.
So I'm not liking to be the guy screaming past you just to wait for that next red light. It's idiotic in my eyes as well. That and I don't like SUVs. I prefer more fuel efficient vehicles. :)