I have the Phoebus in the first PCIex1 slot too. Already set my case up so that a fan is manually pointed towards the GPUs and an exhaust fan is on the side panel - means the temperature of the components isn't too hot but I'm still going to watercool the GPUs.
Helps if you have a backplate on the GPUs to dissipate the heat but could affect the alignment of the soundcard above it.
Intel Core i7-2600K @4.6GHz custom-H20 | Asus P8P67-Deluxe Rev. 03 | Asus ROG Xonar Phoebus | POV-TGT GTX680-SLI U.C. | Corsair GT 2x120GB (RAID0) | Corsair Vengeance 4x4GB @1866MHz | Corsair HX850 | HDD1: WD C.B. 2TB (games/programs); HDD2: WD C.B. 2TB (Fraps) | Win7 Home Premium x64 SP1