Anything that has any kind of (even basic) transparency has to be handled in a completely different way in the engine. Alpha masked will be somewhat lighter than a complex glass material, but it’s still rendered in a separate pass, it still has to be sorted by depths on the screen, etc. 2k maps are fine, especially if you reuse them a lot.
As far as foliage go, there is no silver bullet : it’s expensive and hard to get right. You need transparency, and geometry, and LOD so that you have detailed objects nearby (500/1000 tris) and very light objects from afar (maybe 10?). Read a lot about it, take a look at how the samples are made, how content on the marketplace is done, etc.