Cool test, but I never quite understand the point of such testing. A lot of, if not most, parts do in fact require additional tuning to realize the gains/benefits. Without adjusting the tune to better suite the part, you may not be showing the true potential of the part. I realize this makes it more difficult to do a direct comparison as there are many variables in the tune, but its the only real way to show what a part can do.
IMO, a better approach would be to lay down a good, maximized tune with the stock manifold and do 3-4 runs to show consistency. Then switch to the cosworth manifold and re-tune the car to the same degree of aggressiveness and do another 3-4 pulls. If you can see clear, repeatable gains in such a comparison, and the tuning was done by an unbiased party, you can be pretty sure the gains are a result of the added part.