[stephen wilson]

Downshifting while LF braking is generally reserved for racing-style non-synchro transmissions. I don't know about the Boxster specifically, but shifting without the clutch in a racing environment tends to be very hard on a transmission not built for it.

This is one of those hotly contested subjects, but some alleged benefits in a road race context are: quicker transition from gas to brake, the ability to work the brake against the throttle to stabilize the car mid-corner, and ( the biggest one to me ) not having to do the H&T "foot dance" while braking. your Left foot can focus on braking alone, and your right on "the blip", and picking up the throttle after turn-in.
One down side is that you are more likely to stall the car in a spin.