I have never used GPUImage on iOS. Are we limited in resolution to the Metal/OpenGL ES texture size limit of a device with it? Can it work with extra large images?
The big problem with iOS image handling has always been the pitiful amount of RAM on the devices. It wasn't until relatively recently (iPad 4) that the iPads even had enough to reliably handle images in the 10-15 megapixel range without being killed by the memory watchdog.