I have the Shout blu-ray and I thought the picture quality was very good. Its definitely a worthwhile purchase as you get 2 discs, theatrical and extended cut. There is a review on blu-ray com.
Link
I've been waiting for the Warner Elvis titles as well (especially "Girl Happy", "It Happened At The Worlds Fair", "Spinout" and "Speedway"). I think that the Warner Archive have got such a big back catalogue, there's so many films to choose from and the Elvis titles arent a priority for them...
Studio Canal have released quite a lot of catalog titles on UHD over the last year or so, so I think there's a good chance that some of the titles just announced for 2020 will be UHD. Flash Gordon's a no brainer for UHD, its a popular film.
I have the French release, havent watched it fully yet, just briefly skipped through it. PQ is better than DVD, quite good, but not outstanding. Aspect ratio is correct 2.35:1, unlike the German release which was cropped. French subtitles can be switched off.