I have this set up. With an AEBS with an external HD connected via USB and it is fast to back up stuff. Of course the 1st backup with is rather long because it backs up alot of data, but the others are fast. In fact, I dont even realise that my iMac is doing the TM back ups.
Also, I dont notice any slowing in the speed of my internet connection. I dont see why you shouldn't do it