500GB spinning disk became a performance bottleneck after switching Mac Mini from server duty to desktop duty. I purchased the dual drive kit for the flexibility of adding a second SSD down the road. But for now, the single drive setup suits my needs perfectly.
I'm writing this review on the Mac Mini with a new Samsung 850 EVO SSD... So, it went well!
Take your time as there are a lot of small delicate components. Some people say this can be done without having to move the logic board assembly (slightly) out of the case... I was not able to accomplish this. But I think I only moved the logic board about 1cm to obtain the clearance needed to perform the drive swap. I also used the opportunity with the case open and many parts removed to grab some canned air and clean out about 7 years worth of dust.