The Mill will do very well for DSP work up to a limit of data parallelism. If the necessary SIMD is greater than 8 or so then a GPU-like architecture will do better, albeit at the cost of having to do GPU programming.
Of course, DSP covers many quite disparate loads. For example, if the load requires double or quad floating point (GPUs are notoriously bad at DP and don’t have quad at all) then a Mill will have getter performance than a GPU and lower power than an OOO CPU. In contrast, if the DSP is really not much more than a microcontroller (for example in audio work) then a Mill is overkill.
No simple single answer, sorry.