Does coffee have the ability to dissolve less sugar into it than water?
I will agree with Ivan Neretin. That being said, just because other components are dissolved in solution does not mean that solubility of other components will be affected. (i.e. if you dissolve sodium chloride in water, the solubility of sucrose may or may not be affected). If you have some spare sugar, a scale, coffee, and a little extra time, I'd recommend playing with it a little bit. Look at the literature solubility of sucrose in water, make a known volume of coffee, mass a fair amount of sugar, and see how much you can get to dissolve. Do a few unit conversions and compare the value you get to literature. Could be a good weekend experiment.