Another
interesting read, from Rene Herse's blog. Key fragment might be :
Above 25 mm, the width of your tires are won’t change your speed on smooth pavement (at least up to 55 mm wide tires). On rough surfaces, wider tires are faster.
EDIT
There's also this comparison of Marathon greenguard 32mm to 47mm. Rolling resistance is best at 37mm (1.5"). Albeit on fairly smooth surface, and differences are rather negligible. Interesting read as well.