GPU render imac 5k

Im about to buy the new imac 5k with AMD radeon r9 m290x. But when i go to the Apple shop where i can test these , i cant select gpu render. But the AMD gpu is supporter with the new update of blender .85

Hi, it is because OpenCL is not supported on OSX, maybe next version > http://www.blendernation.com/2015/07/16/more-news-from-amd-opencl-support-in-os-x-el-capitan/.
The iMac use mobil parts so don´t expect to much performance, around 50% of a regular R9.

Cheers, mib

thanx for quick respons but openCL is supported with the latest update so that shouldn’t be the problem. And i know i didn’t expect that it was the best in performance but at least it should be better then the cpu (also mobil part) i think.
-R