Which ARM core is best suited for DSP applications?

Hi,

I am starting work on a project that would require a processor to interact with a custom chip that we have designed and also with a robotic arm. The processor will be programmed to control the robotic arm based on the inputs from the custom chip. The inputs from the custom chip would be required to undergo certain DSP operations and then run some control algos. This will all have to be real-time. I looked at the offerings by ARM and would want to pick one of Cortex-R processors (R4, R5 or R7). I am not sure if something.as powerful as Cortex-A would be needed for this.

Can anyone share their experiences using Cortex series of processors for any similar application that involved real-time DSP so that I can use it as a guiding metric? I would be very grateful.

Thanks

Gaurav