The main difference between a controller and a DSP is the fast working floating point unit from a DSP.
You need often floating point calculation for more precision in some filter types.
A very fast controller is always to slow to calculate floating point in filter-routines. I do some expiriences with 8052 (33MHZ) but its to slow.
In the next time i will check the Cygnal chip. When you using Fixed Point the filter has a high level of noise !