As long as there's no moisture issues with the block wall and the plywood I don't see a problem. It's actually legal to put a service panel directly on a block wall as long as there's 1/4" of air space behind. That's why a lot of panels have small bumps on the back of them....to hold them off the wall. I wish I had my code book with me....I'm not sure about the 150/200 amp issue. Something doesn't seem right there.
