I've visited WDW the last week of January before and it was a pretty good time to be there. The weather can be pretty cold in the morning but, at least when we were there, it warmed up to nice temperature in the afternoon. We wore shorts and sweatshirts to the parks in the morning - and froze - my legs were actually numb. But by the afternoon we had put our sweatshirts in the lockers.
The crowds are basically non-existant. We literally NEVER once waited in line, we walked directly on to every ride. The park hours are much shorter but it won't matter, you'll be able to do everything you want to do - twice, and still have time to spare.
The only thing that's a bummer is that the crowds are so low that it is almost a bit weird. Not that I want to be fighting through crowds of people, but it's strange to be at WDW when it's almost empty. Also, a lot of the food carts aren't open which adds to the "ghost town" feeling - but there isn't a shortage of foods and snacks.
If you like the water parks, January is definately not the time to visit WDW.