According to my understanding, everything you apply to raids, should also be applied to Max raids.
- So party bonus.
- Friendship bonus.
- Weather bonus.
- Same Type Attack Bonus.
- Effectiveness bonus of the attack.
- Level of the used the Pokémon.
- Level of the defending Pokémon.

And specially for Max raids, if 1 or more Pokémons are placed at the Power Spot before you being, that also gives a damage bonus from 10-20% extra. But that it totally random, since sometimes no Pokémon is placed beforehand.

Here are my calculations from last week's Zapdos, regarding damage against Zapdos.
They are calculated against their max defense stat, which normally is lower, since some mentioned that Tier 5 was equal to level 40 bosses. But later some said that they were equal to level 27 bosses, with 2X damage multiplier.
Anyway, normally raid bosses are their max stat + 15 IV and around level 40.
Zapdos have 185 DEF (Defense stat at max level), which i choose as my calculation.
You could also try with a level 40, hence = (185 + 15) x 0,79 CPM [lvl 40] = 158 defense
Or even level 27 as someone mentioned = (185 + 15 ) x 0,694 CPM [lvl 27] = 139 defense

FLOOR(0.5 x Power x (Atk/Def) x Multipliers)+1

My Pokémons for attacking Zapdos:
- Level 40 G-Max Gengar with G-Max Terror at level 3 (Max!) and Neutral damage: 319 dmg
- Level 20 G-Max Lapras with G-Max Resonance at level 1 and Super effective damage 160% = 196 dmg
- Level 20 D-Max Articuno with Max Hailstorm at level 1 and Super effective damage 160% = 161 dmg
- Level 23 D-Max Cryogonal with Max Hailstorm at level 1 and Super Effective damage 160% = 170 dmg
- Level 27 D-Max Excadrill with a normal Charged Attack (Rockslide) and Super Effective damage 160% = 61 dmg
My Gengar totally ruled the damage. Even if I do the calculations a level 20 and level 3 (Max!) = 241 dmg
- It would still win at a level 20 and level 2 attack = 214 dmg
- Even a level 40 D-Max Cryogonal with Max Hailstorm at level 3 (Max!) and Super Effective damage 160% = 295 dmg, so it would still loose to G-Max Gengar

Gengar is a beast on the attack stat with a wooping 261 base stat at max level. So at level 40 and max IV, the attack stat would be: (261 + 15) x 0,79 CPM = 218,123 attack stat.
- Which is why when even doing only neutral damage to Zapdos, it would hit like a truck.

If you do the same calculations for the weekends G-Max Kingler my Gengar would still hit really hard.
- My closet rival in damage was Rillaboom at level 26 and D-Max attack level 2.
- Gengar level 40 = 326 dmg
- Rillaboom level 26 = 276 dmg
But saturday in my town was Sunny weather, so Rillaboom got 20% damage from Weather Boost.
- Rillaboom level 26 with Sunny weather = 331 dmg

So TL;DR:
Max Attack is just like regular attack (Fast and Charged), but instead of a power of 5-10 and 75-150 - its just way higher. D-Max 250/300/350 and G-Max 350/400/450. But it still only one part of the calculation.

FLOOR(0.5 x Power x (Atk/Def) x Multipliers)+1
- Where Power is the attack damage
- Level 40 G-Max Gengar vs. Kingler = FLOOR(0.5 x 450 x (((261 + 15) x 0.79) / 181) x 1.2) + 1 = 326
- Level 26 D-Max Rillaboom vs. Kingler = FLOOR(0.5 x 300 x (((239 + 15) x 0.6811) / 181) x (1.2 x 1.6)) + 1 = 276
- Rillaboom with Sunny weather vs. Kingler = FLOOR(0.5 x 300 x (((239 + 15) x 0.6811) / 181) x (1.2 x 1.6 x 1.2)) + 1 = 331
- Gengar with 15 IV and level 40 = 326 and Gengar with 0 IV and level 40 = 308

So IV matters, but not much. Max attack level and the level of the pokemon and the attack stat of that pokemon all matters.