I agree that the design of the Blichmann false bottom makes me think it might not work well as a boil kettle because it looks like it would be too restrictive. I do have a screen-type false bottom in my 15G boil kettle, and most of the time I don't have a problem as long as I am making 5G batches. On a few occasions when I have made 10G batches there is a phenomenon that has made me hesitant to continue making larger batches with the false bottom in place.
On my first 10G batch, the kettle had about 12G of wort in it and early in the boil I noticed that the activity in the kettle had slowed down quite a bit but the level looked higher than I thought it had been. After a few seconds, the wort level in the kettle suddenly dropped a couple of inches with a big thud that shook the deck. At the end of the boil, I saw that the false bottom had a large amount of break material/proteins, etc. that had built up under it and they had effectively plugged the holes in the screen. My hunch is that a pocket of superheated wort/steam had developed under the false bottom and lifted the rest of the wort in the kettle. It seems like a really dangerous situation and I'm sure glad that it wasn't more violent.
The next few batches went without a hitch, but they were all 5 gallon batches. Then the next time I did a 10G batch, the same event happened again so I knew that it was something that mainly just happened with large batches on my system. I don't make 10G batches very often, but for me hop bags are a much safer option.