Watched 5th Element 4K disc last night, I did notice more detail and some more color, but not a huge step up from the blu-ray. This is probably going to be the trend for older movies, unless a new restoration and new scan is done for the 4K release there are not going to be big improvements over the blu-ray releases. At least there was no crushing of blacks like there was on the Serenity 4K disc. You can only get as good as the source material, so at some point more resolution is pointless.
Looked up some info and 5th Element used the same 4K scan that was done for the blu-ray release, and Serenity is an upscale from the 2K scan done for the blu-ray, so that's why that one isn't really an improvement.