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I got similar spec listed, 4.0ghz i7, 8gb ram, 1tb SSD, M395X 4GB and I just replaced the Apple RAM with the stuff from Crucial : http://uk.crucial.com/gbr/en/imac-(retina-5k,-27-inch,-late-2015)-imac17,1/CT8017545

It's running along with 64gb ram now.

I run multiple VM's with Servers / SQL Server running in some, so memory headroom is very nice indeed.

Thats insane memory but with all that VM usage, definitely worth it for you! I have the same exact machine, only 32gb RAM and the 512gb SSD. I hate to say it, but I only use mine for scholarly research and Word. lol- But I snagged it on Craigslist for about $1600 including AppleCare until 2019, so how could I say no to that. :)
 
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