Goal Semiconductor has a 8052 based controller with a real MAC coprocessor.
It is called Versa 1.
Also there is a Versa MIX (VMX1020) with MAC including Barrel Shifter
Both have RS485/422 interface , ADCs and a lot of goods.

On the site www.goalsemi.com there are also other 8052 like processors.
I thing they are good enaugh for low speed FIR or IIR filters


Cristian