You're not supposed to "mash" with only roasted grains according to a variety of sources (e.g., this). What comes out if you do? Is it only starches, since no enzymes were active?
It's not really mashing if there are no enzyme present. It's just a big steep. But for the sake of argument...
Yes, it's mostly starches. Depending on the speciality malt being 'mashed' in this case you might get some fermentable material but it is mostly non-fermentable material. This is how they make Sinamar. Carafa is 'mashed', boiled and fermented, then it's concentrated down. There isn't much fermentation (only a few points) in that case.