I have a modded Cayenne Turbo now and I haven't bought into this "upgrade". These IPD plenums are promoted around all Porsche models - claiming 30+hp & 30tq peak power increase over the T intake setup. Someone on the 6speed forum inquired with IPD themselves about their HP claims, and their response was that it is more for durability because the plastic ones are prone to cracking.
Basically what I'm saying is, the potential benefit you'll get from adding a splitter or changing to a Y style intake, isn't worth the trouble. The one dyno we saw on a 500chp Cayenne Turbo was like 4hp at peak power.