The position of the tire relative to the control arms won't change as you lower it. If the tire isn't rubbing on the control arms now, it's not going to rub on them when lower.

You could very well rub on the fenders with the new springs installed. I'm running 225/40R18 with about a 2" drop and have rubbed on the inner fender on bumps. Since your tires are taller and wider, you may run into trouble there.

As for mixing tires. It is sort of a no-no. Yes, they do need to be the same rolling circumference. Just because all 4 have the same size printed on the side wall doesn't mean much. There is a lot of variation in size between manufactures. If you want to test this, find yourself a straight and level section of parking lot. Mark a chalk line where each of the tires contacts the ground. Drive straight forward ~300 yards. Do all of your marks point in the same direction? I would guess no. There is some slop to accommodate this within the Quattro system, but there will be some binding somewhere.

 

I think I read the tread thing in the owners' manual

 

That helps a lot. I'll have to go with different springs which means I'll have to put off lowering the car for a little while. thanks for the help.