Kim - I don't think a bigger top mount affects spool, although it will help power somewhat. As it's placed after the turbo, it can't very well add more air to it, only to aid efficiency by being free flowing (ie don't buy cheap imitations).
Rob - that's also what I thought, their tests seem skewed, although they tested on the road and the dyno supposedly. They didn't even bother testing the AVO, because they simply didn't like the look of the design?
Anyway - If Clark Turner says PW is better and makes 10whp more, it's better and makes 10whp more.
To answer the original question, the STI topmount is crimped and won't hold boost as well. It's also a different shape to your stock one and simply installing the STI without a new shroud to optimise the airflow through it - you'll probably be worse off.
On the other hand the PW allows you more boost, comes with a purpose specific hood shroud and I can tell you it works. Take it for a few 3rd gear pulls upto 170km/hr, pop the hood. The cold side is actually cold! It's absolutely fantastic