Your sly remarks hide a hidden agenda
If my "hidden agenda" is to state that there is more than one way to skin a cat, then many of my posts in various threads clearly show that the 'agenda' is not 'hidden'.

I may have an 'agenda' of stating that 'wrestling' a '51 into doing what it was never intended for makes no sense, but that 'aganda' is by no means 'hidden' either.

The only real way to know if it is a better decision is to do a cost/benefit analysis.
I totally agree

The price of this coprocessor chip is $20 with a 700% increase in speed. It is IIC interface so there would be no problem adding it to your slower CPU.
That does not compare favorably with a $5 ARM where you even save the '51, but again, as I stated in a previous post in this thread there MAY be a valid reason to use a '51 (although I see nothing a RD2 can do that is out of the ordinary).

To avoid a lot of flames I declare THIS IS NOT TO INSULT THE OP, just an observation: I do believe that the OP chose the '51 without considering the problem he ran into before deciding to use a '51.

Erik