Rayquaza Investment?

If you don't anticipate getting another one that's better soon, then invest in it when you have a need for it. Need it for PvP? Need it for Palkia raids? (Not yet, you don't). Need it for gym clearing? If you can't figure out what you will use it for in the near future, leave it alone - don't pitch it, but don't invest hard to get resources (RC, dust) into it until you know what you'll get for that investment and if it's worth it.

That range tells nothing. Check your leader's appraisal. If you see that its attack is not poked out, it's usually useless.

Depends on what you have. If you have a lot of other dragons already powered up, i'd hold off. Rayquaza isnt as good as it was before, given the altered effectiveness modifier making the gap between ray and other specialist counter's much larger. As a gym attacker, theres many better options that dont require rarecandy as welll

If you dont have any dragons, i'd say power it up, but only if you have no other dragons. I'd also wait to see if Bagon gets a community day in febuary, as dragon wont be good until palkia comes

I'd invest in it when you need the best dragon attacker and not a moment before. The next time dragon attackers are needed will likely be Palkia raids and Rayquaza will be a great asset to your team for them.