The Low Risk "pool" policy has read and write set to enable by default. The "Low Risk" policy implies you will "trust" those entries specifically should the scanner attempt to access. With that said, you would normally uncheck those two entries (read and write). Make sure that your organization is willing to assume the risk for excluding any item under the low risk pool, with these two items unchecked. The Microsoft article also mentions this. Excluding the Exchange entries will allow the Exchange application to run more efficiently since VirusScan will look at files that are changing and attempt to protect you.
Watch the video TU30285 located on McAfee KnowledgeBase - Understanding High-Risk, Low-Risk, and Default processes configuration and usage.
Hopefully this helps.
I'll also add that a high memory utilization condition doesn't sound like the fault of misconfigured exclusions. I've seen issues with program crashes or high CPU utilization, but not memory. Not that it couldn't be, but it wouldn't be my first inclination.