It is probably impossible to compare the two because you would not get any advantage if you used the same cam profile for the two camshafts. The lifters do not make any difference in terms of the valve timing or lift. That is entirely determined by the Camshaft. The difference is that roller lifters allow more radical cam profiles without experiencing severe wear or friction that would otherwise make a particular profile unuseable. To try to compare the most mild flat tappet with the most radical roller would not really tell you much except that there is a wide range of power variables.