funny thing, I checked the forum because my CoD4 performance on Unibody 2.53 was also a bit disappointing. I am running it on OS X.
The weird thing is, when it starts to lag, I would minimize to monitor cpu usage through istat, and the cpu isnt maxed out (around 70% per core).
Also, when CoD adjust automatically to the optimal setting for my config, it sets screen resolution to 1024x768.
I thought perhaps my hardware was defective. I am also having issues with sleeping, as I find many other people do. Sometimes, it would just wake up from sleeping by itself, then try to sleep again, and wake up, repeat until I shut it down.