A dedicated color internegative film did have an upswept shoulder that fixed the toe problem evening it out, and the mid scale was the standard 0.6 of negative films. And thus, it made an excellent positive. In fact, since the neg-pos system could produce Dmax values of 4.0 or hither, the transparencies produced were brilliant. No E6 (or any reversal film) can produce a Dmax higher than about 3.0.
For evidence, look at any E6 film and then look at the Vision Print Film for Dmax comparisons.