I agree with you assessment in that with two or three turned on, they'll be weaker overall, however, with them all turned on for example, the only one that really seems weak is the dash vents. The floor and defrost vents sound like air is really flowing while putting your hand in front of the dash vents you can feel only a trickle of air coming out. It's expect them all to be fairly equal albeit slower than one individually by itself, not where one is so slow that it's virtually useless while the others are blasting air. But maybe I'm just over thinking this. I've got go in to the dealer for a recall and retrofit soon, so I'm going to ask them.