I don't think it is related with VRAM. I have dual 6GB GPUs, and having no difference between 2K and 4k textures.
Enabling and disabling Virtual Cockpit shadows makes a huge difference for me. Much bigger in BN2 than any other mod. And that is with 2048 pixels shadow map. Im not sure if the VC model has too high poligon count, or a shader issue. One thing I know BN2 VC does not seem like more detailed than TBM900 or Zibo737. Systems simulation is not as detailed as those either. It does not make any sense me getting 10-15 FPS less than those mods during day time, under same graphic settings.
That FPS issue, combined with badly tuned sounds, missing exterior lights and engine management having almost no effect on flight performance (manifold pressure, prop rpm, mixture settings, intake heater settings do feel off and have minimal impact) I got a feeling this release is an early beta, if not alpha of an unfinished product.
To be honest, I loved the inital impressions of flying this iconic aircraft. But after using it for a while, I started noticing those small issues and they started to add up. Now, I can't help myself to compare it with my go to twin engine mods, especially Carenado B58 with rep, and AFL KA350. It fails to satisfy half as much as those mods, although I know each aircraft have different visual, acoustic, and flight characteristics. It just does not feel as alive as those aircraft. :\
I hope those issues will be solved asap, and the simulation rendition will be further improved.