iFly-747 is being loaded with about 700mb VAS remaining and within 5 minutes it goes down to 300 VAS and the OOM signals are heard.
You mean the airplane eats up 400 MB in 5 minutes AFTER it has being loaded ? That sounds like a memory leak to me.
This is not something new and was always the case for the two greatest monsters combined at one point: FSDT-KJFK + iFly-747 (note: the performance is ok! 17-20fps, but VAS...).
FSDT JFK is fairly LIGHT scenery, not nearly a "monster". The problem is likely the REST of the N.Y. area, instead.
My settings are not maxed but it doesn't really matter with VAS, except for one setting that makes it possible to take off in an iFly-747 from FSDT-KJFK (it helps in general), and it is the Global Texture Resolution setting which I had set to "High" (rather than "Very High"). Then of course I restart in flight to free up the VAS.
The Global Texture Resolution won't save you that much VAS. It's more like a VRAM issue, which also has an impact on VAS, if you use DX9, that's why we always suggest to try DX10 (and JFK V2 is fully compatible with it), because it will save you VAS when you need to load lots of textures.
However, the settings that consume the most VAS are, instead, the "Level of Detail" of the scenery, the Autogen Density and the Scenery Complexity. Acting on these, will save you WAY more VAS than lowering the texture resolution.
Another note: PMDG-777 being less VASy can take off from JFK, with awesome VAS leak but it can. As for yesterday's flight, I revised it to KBOS(FT)-PHNL and took off alright.
If the iFly 747 consume MORE VAS than the PMDG 777, I'd say there's really something strange about it.
Now to the point: being a bit tired with VAS issues I am considering going to P3D. I will keep FSX (no panicing really) plus P3D.
Question: is P3D-V2.5 really that better in VAS usage? I hear this on various forums but how serious it is? Would it guarantee me an ifly-747 take off from FSDT-KJFK?
I think its main advantage is that it DEFAULTS to DX11 and doesn't even run under DX9, that's why it's likely got a better reputation for better VAS optimization. This, and probably some bugs that has been fixed along the years. But a more fair comparison would be against FSX under DX10.
If you are not using DX10 with FSX, because you have some addons that runs only with DX9, the issue will be the same with P3D 2.5, just that you cannot switch to DX9 there.