Why's so hard to understand that whp is what counts here?
The WRX has a speed limiter. That's why it can't go any faster. It can do lower 150s without the limiter. The gearing of the STI really isn't that much of an advantage because 6th isn't geared much deeper than 5th on the 5MT.
If the STI was truely the faster car, we'd see it in the acceleration numbers. From 0 to 120mph, both cars post very similiar numbers, many of times of which fractionally favor the WRX.