Those weren't slant-parallelogram, just drop-parallelogram. The first-gen Rally is one of the most beautiful derailleurs ever made IMO, but it definitely isn't slant, it couldn't be, Suntour would've sued the crap out of Campagnolo at the time for that.
And why would it need to shift better? Cause all I've ever heard is that it's clunky and shifts like crap on anything but a straight block, which not everyone uses. I have no first hand experience with one, but would gladly try one if given the opportunity.
-Gene-