He did pose that question when he asked
"for the record I've noticed since the addition of the 15mbps tier my normally solid 7mbps has dropped about 20 to 25%.Any similar experience? Thanks,Bill"
But since I do not have that speed package I can not answer it, because I have not experianced any slow downs only an increase.
The network is designed in a "priority" type system. Obviously voice wins, then internet. Then its a "first come first serve" type basis, with slow downs going down the line as the pipe gets clogged. A simple example is say im using 15mbit and your using the max 7mbit and there is no more bandwidth. Then somoene else logs in and uses 5mbit.. Well say we each lose 2.5mbit, of course that would drop you to 4.5 and me to 13.5 so I would not notice the slowdown nearly as much as you. So its not so much they are "takeing" bandwidth away its just that if there is only 100mbit on the pipe, (not the case) and its divided by customers, those with the higher speeds will not notice any slowdown as much as those on the slower speeds.
So not having the lower tier I can not really judge if I have noticed any slow downs, however it really depends on the node you are located. Some nodes (like mine) have very little traffic, if you live in say a apartment complex which has a lot of passings for a single node, you maybe a bit slower, specially if its like in a college area (where p2p type stuff is big)
So its really on a per node basis. Ive seen nodes with 250+ modems on them (we usally turn them in to try to get them split) and Ive seen nodes with 10 modems. Obviously the least modems in a node the better your response.