Snorlax and not only

HP = stamina x CPM

CPM is the CP multiplier stat which is based on Pokemon level, and at level 40 is 0.8 I believe, i.e. a level 40 Pokemon's actual HP should be 80% of it's stamina, so the difference of 15 stamina IV at max level would be 80% of 15, which is 12.

CPM is the same for all Pokemon of the same level, so it's 12 HP for all level 40 Pokemon.

On Snorlax this difference of HP would be one of the least useful in terms of relative increased time before fainting, due to Snorlax's already high HP, while the two stats that it's lower in, would benefit it more.

As for powering it up, save your candies until you have enough to power it up to where it'll be usable for your own situation. You don't want to power it up half way and on the off chance find a Snorlax that has higher CP and an equally useful moveset, because then the candies you spent powering up the first Snorlax half-way would be better used on your new one.

But once you have enough, it's worth powering - think of all the hoops you need to jump through to get a better Snorlax, all for a fucking 12 HP increase at most.

It has to have better IVs, and even though it's total IVs aren't high yours has the best distribution with what it's got, an equally good moveset since yours is the best at defending (which is 1 in 6 or 1 in 3 depending on your opinion of Hyper Beam), plus there's finding the Snorlax in the first place, whether that's in the wild (which will reduce your odds of getting one with better IVs), or from an egg, which requires you to find a 10k egg first, and that'll take longer depending on how fast you can clear your eggs.

It's not just a question of the pokemon's max HP, as the IV% also affects the maximum CP a pokemon can obtain. Your pokemon's IV% is around 65%, so to be honest I wouldn't spend candy / dust to power him up, despite the fact that Snorlax is very rare to obtain.

If you're not worried about finding high IV pokemon, you're more than welcome to power this Snorlax up, but the IV% isn't very good, and the maximum CP is going to be a lot lower than its potential. For what it's worth, ZH / BS is the best moveset for gym defense, but this pokemon is going to be lackluster in terms of CP compared to other higher quality Snorlax.

The way I typically deal with these situations is to wait until I have all the candy I want to power it up to the level I want. If, in the process of getting the candy I don't find a better one, I go to town once I get the candy.

Most times though, I end up finding a better one and the low CP ones keep taking a back seat...

A lot of people will tell you to wait for a better one. Chances are the "better" one will have like 30 more cp(80ish IV from egg) and a mismatched moveset....and take weeks/months.

If you find a good one later, awesome! You will have two.

Solid advice to hold off till you have the candy though.

Dont listen IV fanatics, its great snorlax.

Just collect candys and dust and wait for better one and when you have enough resources to power this one nearly up to your level, then go ahead if you havent found better one.

I would say power it up, of course. Don't even need to wait for all candy. Cause if you find better one, which is unlikely, considering probability of getting best moveset AND high IV, i wouldn't count on that, you would still want both of them powered. 2 Snorlax is better than one indeed :)

I can't believe anyone would tell you to power this Snorlax up. Level 5 to mid-level 20 to 30 is a huge candy requirement which I'm guessing you do not have enough for. How badly do you really need a high CP Snorlax at this very moment. No matter how unlikely it may seem you will find a higher level Snorlax.

Collect enough candies to get it to a decent level where it will be truly usable (25?) the power-up in one go. If you haven't found one with a better moveset and higher level by the time you've collected those candies go for it. The IVs are perfectly good enough if you don't find one that's better in all aspects.