Ideally, you want to limit your upload bandwith so that your tcp control traffic isn't impaired (i.e. when you initiate a connection to an http server somewhere you send a tcp SYN, receive a SYN ACK, and then you send an ACK, at which point both the client and the server come to the understanding that a tcp connection exists). In other words, if your upload bandwidth is completely saturated with outgoing p2p traffic, your effective download bandwidth will decrease.