Had taken this route on last Saturday. All i can say is, this is one of the best drivei had ever been to Kerala.
Started from Bangalore ( Marthahalli outer ring road) at Saturday 4.30 am, took the Silkboard - Electronic city route. I didn't take elevated highway. I had a fantastic drive till Dindigul keeping an average of 100 kmh. Took the turn at Dindigul and followed Theni, Kambam, Kumily. Roads are very good till Kambam. But little rough before and after Kumily. But easily drivable. Took a turn at Mundakkayam and followed Erumely - Plachery - Ranni - Pathanamthitta - pandalam route. Reached home by 4.30 pm, exactly 12 hours including 2 hours break.
Tomorrow i am driving back to Bangalore in the same route.
What a coincidence - one of my friends - Viju Nair - had driven from Bombay on the same route to Konni (just beyond Pathanamtitta), leaving from Bombay on Thursday afternoon, staying at Belgaum on Night 1, reaching Salem the next night and onto Konni by late afternoon. He also had a great driving time.
By the way I'd second Ampere on the recommendation of R1 over R2 because the roads are way better. A slightly shorter route (than R1) would be Srirangapatna-Hunsur-Nagarahole-Kutta-Kattikulam-Mananthavadi, and as per the thread I quoted above, the bad stretches of road around Kutta are being redone now, so that would also be an option to consider.
How does the texture of this route change with Monsoon?
There are some small bad stretches.
Srirangapattana-KRS Junction (8 kms) - max speed was 90 kmph and min being 50 kmph.
Hunsur to Nagarhole Gate (around 25 kms) - average speed of 60-70 kmph with an occasional 100.
Kutta Police station - Tolpatty Forest Check Post (4 kms) - average speed of 40-50 kmph.
The road in Nagarhole forest is not really smooth but no potholes. Can maintain a good 60 kmph. And I dont complain the slow section 'coz of the beautiful forest / wildlife.