Do raid bosses have better IVs on average?

Currently if you successfully catch a raid boss, the level of the Pokemon is level 20. Like hatching a Pokémon from an egg.

The IVs distribution differentiate between 10 - 15 Points randomly per stat.

Reference: Data from various catches on Silph Road

Yes, raids produce higher IV pokemons since their stat floors are much higher compared to regular mons. Here are several scenarios:

• Raids = 10/10/10
• Lucky Trade = 12/12/12
• Best friend trade (non-lucky) = 5/5/5
• Weather boost IV (non legendary, non raid) = 5/5/5
• Research Breakthrough = ?/?/?
• For reference: Max IV (100%) = 15/15/15

Weather boost adds 5 levels for all mons. Helpful for Legendaries so you start at level 25 instead of 20 for dust and candy costs. Weather boost does not affect legendary IVs.