True, the light output is lower when the Sun is nearer the horizon but only because of atmospheric losses. Angling the panel so the light still falls directly on it will give almost the same output. Note that 'winter' can mean different things depending on your geographic location. PV panels at the equator for example see equal N and S tilt at extremes.
Cloud cover obviously reduces the available light, the specifications are for maximum light before the panel 'saturates' but how much it reduces under cloud cover depends on the cloud density.
Brian.