piloswine 100% lvl20. Power up?

Wait for Mamoswine.

I would at least to breakpoint vs rayqueza. I mean mamoswine is gonna be the next ice type come next gen anyway.

I would definitely power it up at least to level 30. I also have a 100% Piloswine and I have maxed mine out with no regrets.

If you don't have 6 slotted party for Rayquaza, why not? You could use for a full one month.
I have max mine cause i think is a worthy investment as gen4 he might be king of ice.
If you are conservative, try to pump to certain break/bulk point.

If you're in need of ice Mons for Rayquaza and you have the resources to do it, then sure, why not. If your Rayquaza team is pretty solid though, then I'd hold off.

As others said, Mamoswine is going to have amazingly good stats when gen 4 eventually drops. However, the movesets it is given access to may very well determine whether or not it takes the throne for ice types (imagine if they give it the Ho-Oh treatment and give it no STAB quick moves). So, I'd advise against powering one up in prep for the future right now.