BTW here can I find info on the JADE engine and why we hear that is that good for GC (after all it is meant to play primary PS2 games?)?
There just isn't a lot. I don't think it was targeted at PS2, though. The first game that used it was Beyond Good and Evil, which had a custom water effect in Gamecube, a more stable framerate, and better particles than any of the other versions. The other games off the top of my head were the three PoP games (something they did required destination alpha, so the Cube versions have tons of banding, but still run very well and look fantastic) and King Kong.
From what I've seen, it's a pretty flexible engine that allows for beautiful, soft-looking lighting effects and enormous draw distances, and it transitions smoothly between different kinds of gameplay; e.g. indoor, outdoor, vehicles, platforming, etc. That latter may be why they chose it for Rayman.