If an attacker uses something like Illuminati (or just abuses Tor or another botnet) then they can (and should) distribute HTTP requests across different IPs.

If an attacker uses something like Illuminati (or just abuses Tor or another botnet) then they can (and should) distribute HTTP requests across different IPs. This is pretty cheap to do and I've seen it done before in several places I've worked (on the defending side).