The reason for there being no answer yet "at an official level" is because it's specific to each situation and what's running on your VPS.
Cached memory can be high for many reasons: sometimes Apache needs it, sometimes it's being used for mod_pagespeed processes, sometimes it's used for actual buffers on your sites -- but unfortunately there isn't an easy way to figure out what's using it at a given time.
You guys can try lowering the limit to what it appears to be using consistently in total memory and see how it adjusts and utilizes the resources at that rate. It may also be beneficial to "reset" the limits now and again if in doubt, by lowering/raising back up the limit with some time inbetween for it to take effect. If it appears to be a necessary thing to have to reset it to keep the cached memory line down, you both are always welcome to write to us with any details you can provide on what you have running. In general, though, some site/hosting setups just require more cached memory than others and it's generally a good thing when it's being used.
I hope that helps! Like I said, just contact our support team if you guys have any questions/concerns (as you've done in the past); We're always happy to help