Big level difference between players in Borderlands 2, will there be a downside to the lower level ones?

Question

If I (level 40 commando) join a lower level character (level 17 assassin), obviously I'm a bit overpowered for the areas he's questing through.

Other than perhaps not enjoying the full range of boss fight difficulties, will there be a downside for the lower level character?

Things like:

• Game scales up difficulty more towards my level, meaning that while I might still have an easy time with things, he will have a lot more difficult time than if a same-level character joined him?
• Will he get less XP?
• Will some loot be at my level, effectively locking him out?

From the playtime we had today, all items that dropped was at his level so I guess the last question there doesn't seem to apply.

Answer 1

No, there won't be a downside (except maybe losing some fun because you do all the work for him).

In Playthrough 1 and 2 enemy levels (and their drops) are determined by "area levels", that are essentially fixed. Some areas get their levels raised as quests progress, and whole Playthrough 2 becomes level 50 (also known as Playthrough 2.5) when you kill the final boss - but enemy levels do not ever depend on player levels.

With more players in the game, enemies become tougher by some percent, and there are more of them in some predetermined places - but all that is designed to be within your combined firepower increase, even if you are all the same level.

In multiplayer, XP is shared between players, and if I recall correctly (it was some time since I played), the XP from a kill is first distributed (essentially copied to each player) and then modified by player level (so that if you're too high or too low, you get 1xp). So your friend will get same XP as solo.

See also:
What gameplay effects change in multiplayer?
How does enemy difficulty scale up relative to the number of players in co-op?

Answer 2

I believe that in both playthrough 1 and 2, the enemies have set levels for each area.

What I don't know is how the enemies get stronger. It could be that their levels are higher, or it could be that their levels are the same but their HP is higher and their shields are stronger. I haven't played enough to notice.

Game scales up difficulty more towards my level, meaning that while I might still have an easy time with things, he will have a lot more difficult time than if a same-level character joined him?

I believe that the enemies' levels may change based on how many players enter the game, but I don't believe it's affected by the levels of the players that enter the game.

Will he get less XP?

The XP should be based on the levels of the enemies, so I'd bet he'd gain XP faster by playing with you than by playing alone. (Even if the enemies' levels don't change, you'd enable him to take on stronger enemies than he'd be able to face by himself.)

Will some loot be at my level, effectively locking him out?

I think, as you noted, the loot is similarly based on the levels of the enemies, so odds are you're the one who won't be finding much loot worth keeping.