NVIDIA limits the double precision performance of their consumer graphics cards to 25% of the possible performance. The GeForce GTX 550 Ti has 691.2 GFLOPs  and therefore would be capable of 345.6G double precision floating point operations per second. It is limited to 86.4G though. So for Fermi-based consumer cards the double precision performance is 1/8 of the single precision performance.
Does AMD also limit the double precision performance?
According to what I red double precision performance is 1/5 of single precision performance with AMD GPUs. This means that the Radeon HD 5850 has a double precision performance of 417.6G which is 20% of 2088 GFLOPs  .
I want to use OpenCL with a 64bit Linux real-time kernel. The NVIDIA driver supports this but as far as I know the AMD one does not? Is real-time support planned? I found information online on how to use the AMD driver in a real-time environment. That requires quite a few code changes though. Then again with the AMD card I would get almost a 400% performance increase.
Taking all of the above into account would it be better to buy a Radeon HD 5850 for OpenCL programming since it costs the same and also supports OpenCL1.1?
Is it really so that developers only get consumer graphics cards and
therefore only 25% of the possible performance -
instead of developer card with 25 % price and 200% performance!?
Is it really so that developers only get consumer graphics cards and therefore only 25% of the possible performance - instead of developer card with 25 % price and 200% performance!?
I don't really get your question. I could of course get a FirePro or Tesla card but these cost a lot more money (4-5 times as much for the same performance).
Take these 3 pretty similar AMD cards (number of cores, frequencies, performance, power consumption, ...) for example:
Since I would use the graphics card only for OpenCL computation without a display even attached to it I don't see any reason for why I should spend more money for less.
The double performance is by design. The VLIW units have to work together so the max GLOPS are reduced. 5850 is a nice card. although you can also look in 6000 series.
Only the Radeon HD 6950 and up of the 6000 series support double precision  which is very important to me. Those cards also draw to much power for my taste (ca. 150W is my limit).