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I'm a fat-arse and I've skied my SPK Kaos in bottomless jap powder with no issues at all. The skis you'll be on tend to be much softer than the boots anyway.
I guess a stiffer boot would give you a tiny bit more control over big, heavy skis but personally I've never got to the bottom of a run, wiped away all the pow that has covered my goggles then gone 'aww man I wish I had stiffer boots on'