Why is this even a question? You've already got the saddle and you know it doesn't work. There doesn't seem to be much downside to trying to modify it yourself. Guess you might destroy any value it would have on the used market. I guess the key question is: how much does a used, partially broken-in Brooks fetch on the used market?
Anyway, I'd say: go for it. If it doesn't work, you can always buy a new Imperial at some point in the future. Of course, riding the modified saddle might convince you that the slot isn't going to help and that you'll need to try something else entirely...