Weather-boosted pokemon basically differ from regular ones in that they can appear up to level 35 in the wild (instead of 30) and have a higher floor for IVs (I believe 4-4-4). Yes, this is permanent once caught, but bear in mind that should the weather change (to something that doesn't benefit the type) whilst a pokemon is out in the field then their stats will be re-rolled.
You can tell if something is being boosted because they have a blueish-white swirling halo around them on the field.
Raid bosses can be boosted too. These will be level 25 but do not differ in IVs from normal.
