On the fence with my Raikou

Yeah. It's worth it, even if the attack value is the low value. Raikou is really useful right now.

Maybe when Gen 3 comes out it will be nerfed (by hook or by crook), but for now it's really, really useful.

My best Raikou ended up being 91% (14a/13d/14s), and I’ve taken it to 40 with no regrets at all.

Game press considered Tyranitar, Mewtwo, Raikou and Zapados has the 4 safest bets to power up https://pokemongo.gamepress.gg/safest-pokemon-bets-gen-3

Yes, unless the Raikou has a crummy moveset and you don't have TMs to fix that problem. Raikou is really useful. Note that a 70% Raikou, powered up, would probably still be one of your ten best Pokemon. 91% is nothing to be upset about. Run with it.

Do it, the best that I have will not reach 3300 cp :(

You should go for it and make sure it has wild charge. I made over 40 raikou raids and have never seen one above 1900...my best one is a 1898 96% 13a/15d/15s one. I also have 12/15/15....Then I have one with 14/15/13.
Would you power up 13/15/15 or the 14/15/13 one?

You should. Overall, it is a very strong pokemon.