I have a FSC Lifebook with a Intel 945gm running at 1400x1050. The cube effect - and other kwin effects in openSUSE KDE4.1.x packages run nice and smooth.
I’ve also seen kwin effects running nicely on the first generation of EeePCs (it was called 4G or something? It’s a 7" at least). This didn’t have the cube, but wobbly windows, switcher effects etc. ran nicely, so why wouldn’t the cube.
I’m not aware of any benchmarks. To me they feel about the same performance wise - but this all depends on specific hardware, driver versions, configurations, which specific effects are used etc. Personally I certainly prefer KWin for integration, stability and window management functionality reasons.
I’m not aware of any “significant performance optimisations” for 4.2, but KWin effects are very young and improving all the time - almost on a daily basis.