Wild Willy - The flow rates are not much differrence between the two, the obvious answer is that a 200 cell cat has twice as many cells. But as far as phisical surface area (head on) difference, there is very little.
I do have some flow figures around here somewhere for the tests we did. Will see if i can dig them out and post them up for you. In the meantime, i will take some photos of the 100cell and 200cell cats we have here so you can see the difference

Yes a 200 cell is similar in the way you describe, when held up to the light their is little obstruction.
Kind Regards
Garry
Exhaust Sales