I just realized I have been working with the latest version of ZBrush with MEMORY STRESS TEST constantly switched on. What does it do, and should it be on or off?
Also whilst I am referring to memory, I have burnt out memory on 3 separate occasions using ZBrush, luckily under guarantee. From different slots and risers, so definitely working it very hard clocking up close to 80 hours a week on ZBrush for a few years now, on an old early 2008 Macpro using DDR2 RAM, eeeek.
80 hours a week, WoW man, thats crazy obsessed! I like it! 
I Find it odd you burn through memory that fast… I wonder if perhaps your macbook has cooling issues. I find that laptops of any kind are terrible for 3D work. I burnt out one myself, using Zbrush and rendering with Vue. Never again. I will keep my Keep my “anti-Macism” to my self 
, and just stick to my desktop Bias.
Have you tried Getting a good Cooling Fan Pad for your Macbook? That may save your Ram life.
As for your memory stress test mode, Where is that button located? and In my humble opinion, I would turn it off, no need to push your ram any harder than you have too. I would guess this is something left over from the beta Testing, if you were apart of that, or just something they left in from the testing phase, and Some how you have turned it on via an accidental hot keying. :o