Quote Originally Posted by Sushigaijin
The opposite, actually.

A 4/3 sensor would just use the "sweet spot" of an APS lens, while an APS sensor would record the imperfections at the edges like vignetting and softness. Since the center of the lens is closest to "on plane," it is the sharpest and most accurate part of the lens.

Quality diminishes towards the edges, which wouldn't even be visible on a 4/3 sensor.

Full frame cameras are the most susceptible to lens flaws. The smaller the sensor, the cheaper and easier it is to manufacture a crackin' lens.

This is why the (sigma, rebadged by olympus Zuiko) 70-300 lens is sharper on 4/3 than any of the other mounts it comes in: the 4/3 sensor just uses the good parts of the image circle and discards the rest. It's just like cropping the center out of an APS shot and blowing it up to equal size, without losing any IQ due to enlargement.
No, that is only one aspect of lens quality. Higher pixel density means things like diffraction will be more severe. High pixel densities can bring out the best things in a lens, but it also makes lens problems worse.