That won't change anything. The subpoena will still be for the person who pays the bill at a location where multiple devices may be connected. I doubt that judge will give a subpoena to go look for a computer with a given MAC address at a location. But those worried can just change MAC address every few days.
If you have 2^48 IP addresses (like anyone with a free ipv6 tunnel does), you can assign your device a new IP address every day for 771164319755 years.
