Traffic jams per se don't bother me; they seem part of the game.
Certainly the more players, the more multi-player chaos inhibiting strategy (as in almost all interactive games - I don't see this as being anything unusual about K2 in particular).
I still like it with 5, but I agree that it is more "serious/strategic" with fewer players.
There is the addition of having to plan your moves to take into account end-game turn order. Knowing you'll be on the wrong end of some blocking might require an earlier ascent.
I agree it is more chaotic, though, but the fact that it's so different yet still good with 5 is one of the plus points of the game for me.