That's true: This tip of disabling scenery came up a couple of months before the release of PMDG's T7: Some guy on the AVSIM forums, who had installed masses of photoscenery, was facing OOMs over and over again. PMDG's Ryan helped him out just to discover that disabling all that PS brought back about 1 GB (yes: 1.000 MB!) of VAS to this guy.
Those findings were discussed on AVSIM and have made it into the T7 introductory manual.
I installed quite some PS in recent weeks (freeware Canary Islands, freeware NL-2000, a payware PS for the Balearic Islands; I also had had some MSE, VFR Germany, BSS and other PS before) so I gained those 400 MB. You'll probably need a huge installation of PS to reach between 0.4 and 1.0 GB. (I have 538 layers in my scenery library!)
But flying into and out of London airspace, it will help for sure to disable all those Gary Summons airports you don't need (otherwise they will be loaded during those London SIDs and STARs and fill up your VAS as active scenery). The same is true for the N.Y. airspace and maybe some more (Hong Kong? Flying into YMEN, you may want to disable that huge ORBX YMML, perhaps?)
Anyhoo, "just" 200 MB of additional free VAS can make all the difference when descending into one of those addon airports.