I love the nit in this story. When Holmes says he's never heard of the 'Bruce-Partington' submarine, Mycroft can't believe it: 'I thought everyone had heard of it!' Less than two paragraphs later, of course, he's explaining what a tippy-top state secret those same plans are...

I always thought Sherlock was pulling his brothers leg.

I don't see this as a nit. Think of it this way: "everyone" (I mean it in the sense that it's common knowledge) has heard of the B-2 Spirit or the F-117A Nighthawk, the vaunted "Stealth" planes that the US Air Force has. We've seen pictures of them. People in other countries know of them. But their plans and true capabilities are jealously guarded.

On reflection, this seems to be a retelling of "The Naval Treaty." In both stories, important state documents go missing. In both stories, someone responsible for the plans falls grievously ill from the shock of the theft. In both stories, the thief turns out to be related to the same person who falls sick. In both stories, the thief has a debt to pay and plans to sell the papers for money.