Do we actually have any confirmation that HK will eventually change to the diopter sights for the SP5? One of the images on the product page on HK's website specifically mentions using the same rear sight as the MP5K. Plus the fact the castle sight is the usual sight used for pistols variants. The supply issue does perhaps suggest the change may be made.
Not really a big deal either way in my opinion. Wish it came with diopter, but it looks easy enough to change out. And with the demand of these things so far I'm not gonna wait for them to change something so small.
On the subject of YouTube videos, the "HK Shooting with Derek G" channel has some nice and detailed videos on the SP5. He covers several different aspects of it, including how to change out the rear sight.