I track my base '99 pretty often and I've driven an S model too. Yes, the S will pull better out of a turn but a 987 will outpower that and a 997 will outpower that. Its easy to get onto the horsepower merry-go-round.
The base Boxster has more than enough power so I can learn to drive well on a track and have more fun that I've ever had before and still be able to give myself a darn good scare. When you think that you're going off into the dirt at 110mph, you could care less about your HP and a lot more about your brakes!
In regards to Boxster vs Miata, I happen to own both and had them both at the Streets of Willow last weekend. My experience is that the Boxster is indeed faster BUT 99% of the time it comes down to the driver and not the car.
Here are a few pics from last weekend:
