I have not done around the coast but did ride straight across between those two places last year using a route provided by someone who had done an organised trip, it was very good some of the quietest roads I have been on in India.
I have also ridden up from Kanniyakumari to Kochi along the coast road on a motorcycle and that was pretty busy, I am not sure about all of Chennai down to Kanniyakumari, as far as Puducherry was fairly quiet but I was inland down to the bottom.