Bandwidth is the theoretical maximum amount of data you can send at any one time, usually expressed in Megabits per second. The data sent is split into small chunks called packets, the greater bandwidth you have, the more packets you can send in the same amount of time. This maximum is set by your ISP and will not change no matter how long the cable, in theory you can still send at your maximum allocated speed.
When you increase the cable distance between the router and the socket you introduce voltage drop and noise on the line, this means that packets will be sent at the same speed but may become corrupted before reaching the exchange, meaning that the same packet has to be resent. If this happens to every other packet, each packet has to be sent twice, therefore the rate you can send data is halved. You are still sending and receiving packets at the same rate but some of them are discarded.
The only real way to tell how your set up affects your connection, if at all, is to use a site like this one:
http://www.adslguide.org/tools/speedtest.asp
Try this with your router in both positions and you will be able to tell how much effect the cable length has. Probably not a lot as if your router is permanantly synced to the exchange then the connection is solid.
If there is a lot of loss, you could put your router at your master socket and run a LAN cable from there to your PC.