Good points you have raised.
Frankly, I do not understand how by keeping the numerator constant and by changing the denominator, the ratio can remain the same. This is mathematically impossible, but I think anybody with a penchant for the English language can become an author of technical articles.
An engine will always produce the same horsepower, if it remains the same engine. The brake-horse-power can differ from the horse-power, because, there are differences in the ways one measures them; however, as long as the engine is same in Mark I and Mark II, and as long as Mark II weighs more than Mark I, I fail to see how the power-to-weight ratio can remain constant.
This is just absurd, and I am, perhaps, missing something.
Regarding the change in the sprockets, I think I read they changed the way it was being made. Earlier, it was fabricated, and now, they are simply casting it (IIRC?); therefore, it is cheaper and quicker to produce.