Sorry if I don't answer all your questions ... but it does not lead to anything :-/
What do you expect Nvidia to do? Their Xrender implementation isn't the best, but I don't think there's a performance-crushing bug in particular that's causing this. It happens with other cards, and it happens also on my Geforce 4 MX 440.
Do subpixel rendering using shaders on boards where they are available (FX5200 and up). This way they could also sell now hw to customers which really want this feature.
Sun showed that this can be done and that its quite fast on higher-level hw.