There's a good chance it will be cool (low 70s), and as EuropeanPete already noted, it's been pretty rainy the last few Januaries.

You may do better on the Andaman Sea side (Lipe, Lanta, etc.). Also, the Thai islands are not necessarily the best place to burn points. You will often do better just paying cash for a local resort that offers beach bungalows (and will often include breakfast, wifi, and airport transportation in the base rate, unlike the major chains). If you want an SPG resort, you might consider Naka Island in Phuket, which gets very good reviews, though I would otherwise stay away from Phuket, too many hassles, from airport, to taxis, to large tour groups.