Yes this is a no brainer for mac users really as 90% of their machines are supplied with Radeon cards. OpenCL has additional parralellizing instructions, works equally well on nVidia and AMD cards and benefits the whole community. CUDA benefits vNvidia only and cuts most mac users out. I would purchase the server edition today if it had OpenCL support but I'm not about to buy a new GFX card so I can use Nebula on GPU.
Looking at that document it looks like the porting, however tedious, would be fairly straightforward.
Yes this would be awesome. The CUDA-engine is limited to Nvidia only while OpenCL is an open standard that will work will all GPUs with OpenCL support, including GPUs from Nvidia, AMD and Intel (integrated). And this is quite possible, I use a OpenCL-based vst right now actually: