So just to clarify, your issue is the memory that is being taken up by the Cubemaps and the artifact you are reporting when using the default compression?
I did some testing on my end in a new blank project and found some interesting results. So, the default texture compression and settings that Unreal chooses when you create a Static Cubemap Texture is optimized for the PC platforms.
This is why it compresses using the HDR (RGB, no sRGB), unchecks sRGB, and has NoMipmaps. I took a look at the resource size after messing around with some of the compression settings, and found that if you check the ‘Compress Without Alpha’, your resource size is cut in half. This is also when using the the Default (DXT1/5, BC1/3 on DX11).
Default Cubemap Compression
Modified Compression
Hopefully this provided a little bit of insight into this issue, but let me know if you have further questions. I still need a way to reproduce the artifact you are reporting so I can enter an bug report if necessary.
Thank you,