Easy distribution and installation are huge advantages to iOS Apps compared to Linux. The potential number of users is amplified by factors of millions, mainly because app discovery and install are so easy.
OpenGL Compute abuse using TransformFeedback is possible
Latest devices with A7 chip (with integrated GPU) support Metal which provides CUDA like compute capabilities.
http://www.anandtech.com/show/8116/some-thoughts-on-apples-metal-api
On a final note, while we’ve discussed graphics almost exclusively thus far, it’s interesting to note that Apple is pitching Metal as an API for GPU compute as well as graphics. Despite being one of the initial promoters of the OpenCL API, Apple has never implemented OpenCL or any other GPU compute API on iOS thus far, even after they adopted the compute-friendly PowerVR Rogue GPU for their A7 SoC. As a result GPU compute on iOS has been limited to what OpenGL ES can be coaxed into, which although not wholly incapable, it is an API designed for dealing with images as opposed to free form data.