That looks really good, and there are so many different shades of grey/silver on the GS, that if the top cover isn't a perfect match to your panniers then I don't think it really matters!
My bike is now all ready for the aluminium sided SH35 panniers which match the SH59X, but they are also unavailable until September. I've looked at alternatives, but like you will get the SH58X top box. I will use it with the SH36 panniers I bought to use for checking clearances with while modding the panniers frames and mounts while I waited for the SH35s. I'm touring Spain in August and will be near Barcelona for a week so may visit the Shad factory which is there, and see if there is any chance of getting the aluminium lid from the SH59X to fit to my SH58X!
Here is a pic of the finished frames and custom made brackets, now all nicely powder coated by CBS Engineering in Long Eaton. I can recommend them, they did a great job and were really helpful, just ask for Keith who does the motorcycle stuff if you contact them. I also fitted some tool tubes, and made them lockable with a hole in the mounting bracket plus a stainless eye on the tube lid to take a padlock.
Fred
Your bike is looking splendid and really functional. Good luck with visiting Shad - if the other top fits I may have to consider it too !