Barring any very unique attachments most Thule parts are cross compatible with Yakima parts (and Rocky Mounts too). Basic system is that Yakima bars are round and Thule are rectangle. HOWEVER..... Last I checked Thule doesn't make an "official" Thule system for the 997 because they actually make the OEM Porsche rack (more $$$$). That may have passed now but the official Porsche/ Thule sits inside the flip covers but also has less width vs. the Yakima which sits atop the roof (and secured with the bolts inside the flip covers) but you can put any width bars you wish in case you wanted, for instance, a box and bike racks.

Just don't rule other other systems simply because you already have a Thule attachment.

PS. Only other thing to consider is that all company's are coming out with aero bars that may not be compatible with their own attachments.