I figured I would add my two cents,
I used threatfire 4.7 with Avast 5 and found extreme performance degradation. I gave up and removed threatfire from all of my machines. Threatfire seems to be a dying product anyway since it has been a significant amount of time since a new version was released. I currently run Avast 5 and MBAM Pro (my threatfire replacement). As an experiment I added Panda Cloud 1.3. Vista and Win 7 can handle Avast 5, MBAM Pro, and Panda with minimal impact on performance. XP however suffers severe performance issues when Panda is added. Disabling both the behavioral detection settings helps but the machines still suffer. I'm hoping that Panda improves their product in time and resolves some of the compatibility issues (mainly in the resident protection) though I'm not overly hopeful since XP is outdated at this point. As an aside, it seems that the performance issues on XP are related to a conflict between Panda and MBAM based on CPU usage observations.