I don't think that the lever arm height difference on the new v. old pressure plate is a significant factor. If the arms are longer to the pivots then they give you more leverage and reduce the effort at the pedal a little.
You do want to measure the difference in Marcel between the two discs.
Place them both flat on the same surface and use a straight edge, something like a carpenters framing square and compare the heights of the friction surfaces.a
This will give you an indication of the amount of "spring" built into the disc for the Marcel effect.
You are looking for the same height or hopefully less on the new disc.
Compare the two numbers and remember you were trying to gain at least another 1/16" in travel (.0625") if not more.
An increase in the sprung height of the disc (the Marcel) would be a main culpret here.
I think that you probalby find that everything is nearly the same and the differences could be explained by wear on the old clutch?
The simplest solution is probably going to be to change out the existing slave for a "long throw" which probably will just show in the current listings as the "standard Pantera slave"?
The new clutch is pretty. Do the lever arms show signs of the counter weights being cut off or made without them?