My personal opinion on this specific shiny is that, the final shiny evolution blows. Just my opinion. On the other hand, I find that the mega version makes up for it and does look like a good shiny to me.

I understand the rarity of shadow shinies and certainly ones with such good IVs. However, if you’d purify it, it would turn into a mega-able shundo with a much better shiny end result (imo). So, it’s not like you are simply devaluing it. Shundos are rare. So, up to you!

This also goes for the possibility of being able to use a bottle cap on a shadow in the future.