Is 93% legacy snorlax worth powering up

Depends on how crazy the gym defense scene is near you and if you already have powered up Blissey or not.

Either way I'd wait until you get your final breakthrough Snorlax before deciding. Next week's could be 96%.

Probably.

Depends on your local Gym game (particularly how many Gyms have Snorlax on them regularly), on how much Stardust you have, on how many attacking Pokémon you have "maxed" out (ie: raised to the highest level you need them to be at), on how many Golden Razzberries you normally have, and on how many good gym defenders you currently have.

Personally, I'm not powering up any more gym defenders at all. Did that between the release of Blissey and the Gym game change, back when CP was the most important thing for keeping a Gym. Now I try to put unique Pokémon or Pokémon that people want candies for (Milotic, Bagon, Metagross are the ones I do that with mostly), or ones that are unusual in some way. (Wigglytuff and Clefable are favourites, and I've got a few Ninetales and Typhosion that have Solar Beam just for fun) Most players here have Snorlax and Blissey, so if I'm not the first one on, I usually can't use a Snorlax or a Blissey anyway.

You're snorlax is not legacy. Nor is it special. Everyone has one now.

Tough one, I have a similar breakthrough Lax but also a 100 IV one from a hatch I haven't touched yet. Powered 2 up pretty early on but got them after BS became legacy. I'd say wait, incase you get a better one & power up only if you don't have a decent one already. The gym meta as it is, you certainly don't need one, but you probably still want one.

Ask yourself this question: will you be happy with your investment when anyone with a decent level can punch it out of the gym in 30 seconds or less?

I guess it depends on your gym scene. I have a level 40 BS Snorlax and I can't remember the last time I used it. I either use Blissey, when I know there are people that will actually struggle with her, or I put in something that isn't weak to fighting. Grandevior is a good option. I personally use my level 40 100% Clefable and she racks up a win in almost every gym, probably because people don't switch out their Machamp. This is most gyms around here...

There are not now an unlimited supply of BS Snorlax because it became available again for the month. You have a 93% of one of the better gym defenders, with a good move that a number of people won't have.
If you need some more gym defenders, Snorlax is great. Some of what others said still stands though; Blissey, Chansey, and Snorlax are quickly put on gyms so you might only be using it if you're the one flipping the gym. I've been playing since last fall when Gen2 came out and a good snorlax has been pretty elusive to me so I've relied on off-brand defenders to compliment the trio. DT/B Milotic, RL/SldgB Venusaur, PJ/B Tentacruel, C/IB Slowbro, whatever it is. I also managed to get a good Snorlax from this event and will definitely be powering it up, I'm lvl 36 now and take gyms frequently. I'm not a hard core player so I don't already have an army of Blissey and Snorlax, so this seems worth it.