Penalizing high bandwidth users is kind of an interesting heuristic. It's definitely better than penalizing long connections, at least!
However, I think you're assuming that all the clients are the same. This is definitely not be the case in real life. Also, not all applications that need low latency are low bandwidth. For example video chat can suck up quite a bit of bandwith.
Just to take one example. If I'm the cable company, I might have some customers with a 1.5 MBit/s download and others with 6.0 MBit/s. Assuming that they all go into one big router at some point, the 6.0 MBit/s guys will obviously be using more than their "fair share" of the uplink from this box. Maybe I can be super clever and account for this, but what about the next router in the chain? It may not even be owned by my cable company, so it's not going to know the exact reason why some connections are using more bandwidth than others.
Maybe there's something I'm not seeing, but this still seems problematic...