Why can a system 3x more powerful not get 3x the performance?
Eh, short answer... 3x the crappy programming! : P
Ah, I jest... I don't know anything about Gearbox except they have the misfortune (or, fortune, depending on which gamer you ask, stilted or no) of being partners with M$.
But seriously... they have been working on Halo PC for a good while now, including a full network (and graphics engine) rewrite from what I understand. So, if there is NO co-op (@$%@#) AND the net code sucks... that's, um... bad!