I like the math, Zif. It's a nice way of thinking about the value.
I know you meant 5/6 odds of wounding, not 1/6.
I'm thinking through your reasoning, because it seems incomplete to me. Much, I think, depends on how important it is to remove those cannon. However, as a rough heuristic for calculating the cost of an action, I do like your reasoning. A lot.
I do think there is value in losing 1d3 wounds but gaining 1d3 Steam. I'm not sure I'd value it as a loss, with 7 wounds still on the tank.
Again, I really like this form of reasoning, but I think there's a little more nuance to it.