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In practice the difference between 4/3 (2x crop) and APS-C sensors (say 1.6x crop) of identical resolution (pixel count) is small. The aperture thresholds at which diffraction starts to limit sharpness are probably less than half a stop apart. The other way around diffraction starts to limit sharpness on a NEX-7 (APS-C) and a 16 MP Panasonic m43 cam at virtually identical aperture settings. Of note, the effect at the aperture threshold is for pixel peepers only - 100% on screen viewing is needed.