I agree with you about Innova eliminating some discs (Katana, Wahoo, Archon all fill the same slot), but I don't think the Sidewinder and Roadrunner overlap at all. The RR is way more understable.
I found my first Roadrunner (Champ) umarked on the course. It was indeed considerably more unstable than my Sidewinders. I finally lost that Roadrunner and have bought several more since but none are near as understable as the one I found (which was not very beat). Some of my newer ones are even lighter but they're still pretty interchangeable with my Sidewinders. I'd love to find one of those old flippy RRs.
My Monster is way more of a meathook than my flat top Firebird. My Firebird is my ideal of what a long overstable disc should be while my Monster is a ground-seeking device.