There is nothing wrong with using Quixel. We have a good relationship with them and their software. Our Scifi Hallway demo was an example they provided for the community using their DDO and NDO plugin for photoshop!
You shouldn’t have an issue using 2048 textures, but I wouldn’t use 2048 for every object that is used. Some you can get away with much lower resolutions or by using texture atlas ([one material for multiple objects][1]).
Breaking the geometry up and importing as separate meshes will give you the most flexibility. You can have a single wall mesh untextured that could be used over and over and just apply a different material that suits its needs.
Material Instancing here can allow you to have the most variety with the best performance! This will result in only one draw call even though it is technically different materials. [Take a look at the vids here if you’re not familiar with this very powerful feature!][2]
With regards to decals here is a direct quote from our rendering programmer taken from the forums:
“Decals cost similarly to an
unshadowed Movable or Stationary
light, because they are both applied
in a deferred pass. Covering the
entire ground with them should have a
pretty heavy cost, especially as you
look at an angle where you are looking
through potentially hundreds of them.
GPU costs such as this can be seen
with ‘profilegpu’, so to a performance
test. Decals are under
PostProcessDeferredDecals.”
[Make sure to use the view modes when trying to trouble shoot profiling issues. Here is the documentation to make sense of what you’re seeing and how to make your game more optimized!][3]
Feel free to ask any questions! 
Tim
[1]: http://timpaauwe.wix.com/portfolio#!texture-atlas/zoom/c1i1j/i138x
[2]: http://www.youtube.com/watch?v=sx0BlzGkahw&list=PLZlv_N0_O1gaCL2XjKluO7N2Pmmw9pvhE&index=104
[3]: Viewport Modes in Unreal Engine | Unreal Engine 5.3 Documentation