Welcome to WebmasterWorld Guest from 22.214.171.124
Your IP address method would hurt even a stand-alone domain. I've seen some trickier methods bandied about to reduce spamming; one was to use the Whois database to eliminate multiple domains owned by the same person or company. Another was to compare hosts.
But, if the earlier posts are correct, and the weighting of this factor has already been significantly reduced, then we are already being hurt.