[Brad]

There are a lot of variables involved here.

To begin with, top speed is limited by several factors. Power of course, but the effective "gearing" of the prop- in other words, the maximum speed that the prop could theoretically pull an aircraft. There are "climb props" and "cruise props" which for the same hp, a "climb prop" will provide more thrust (better takeoff and climb performance) and a "cruise prop" will provide a higher top speed, but sacrifice acceleration and climb performance. And of course the drag of the airframe.

And as stated by one of the other posters... but I'll paraphrase. An increase in speed takes a much greater proportional increase in horsepower. An increase in climb rate is more linear. I suspect the power was applied to giving the aircraft a better climb rate instead of a very marginal increase in speed, which of course is determined by prop selection. It is all a trade-off.