I asked a tech rep at Custom that same Q about 18 months ago. He said performance is the same as long as the modified bag of thinset is fresh. If the bag has been hanging around for a while, you'll get better performance by adding the modifier yourself.
That's meant to be an apples-to-apples comparison. An old bag of unmodified thinset that you add a liquid modifier to will give you better perfomance than an old bag of modified thinset.
I have always added the modifier myself. Reasons? More options. Buy the best thinset (yes, that means the most $$) that your store offers. It's only a few more dollars per bag, but you'll notice a tremendous difference when handling the product. I find better thinsets much easier to work with.