I have had pretty good luck following this guide in both FSX and Prepar3D. http://forum.avsim.net/topic/462838-a-guide-to-getting-the-most-out-of-the-pmdg-777/ Definitely pushing the VAS limits at CYYZ, but I'm able to get in and out without OOM ( pretty close to the limit though ). FPS is low, but ...

I saw this too with both KBOS and KMDW. After some troubleshooting, I narrowed it down to it being a result of the date being December 31st. For my system all is ok on any date other than December 31st. Have you tried that? I'm curious if this is true for others?