When you don't set affinity, the WoWs restrict themselves to core 0 and 1. (Unless Blizzard has changed that behavior very recently. I haven't tested it lately.) That's why you see lower CPU usage. Half the CPU is mostly idle. This is worse performance not better performance.

Did you try setting affinity to "all" for every WoW? That allows the operating system to optimize as best it can. That's a sort of baseline for performance.

Page files aren't a problem. Paging (page faults) are a problem. They are not the same thing. You can monitor the amount of paging by watching "hard faults/min" on resource manager (click the Memory tab). With eight gigs of memory I wouldn't expect a lot of paging.