The IOP has 2MB. I always call it the Input/Output Processor, dont know if thats official though. I would expect buffer sizes used here would be fixed, independent of which video is been watched, so its not really important to report its free memory.
Using more memory would probably not make it any faster. SMS is CPU bound, meaning thats the bottleneck. Using more memory will just mean its more for the CPU to do, which it cant handle.