If they put out a METAR saying cloud is 1500' when it's actually 2000', do any pilots actually radio this info back to help the ground observor improve their judgement?
Yep, most definitely PIREP add to the accuracy.
I find the info in the ATIS generally correct, but if I think there's more to it than what the ATIS tells me, I'm indeed doing a pilot report. Especially if I know that there are fellow pilots who are interested in this information on frequency.
ATIS content comes from the same human observer. It should be the same as the published METAR.
The easy way is, if you have access to both surface temperature and dew point, is to calculate the cloud base using that information.
In the International Standard Atmosphere, air cools from the surface up to 11000m at a rate of 2 degrees per 1000ft.
i.e If the tempertaure is 06 deg C and the Dew Point is 04 deg C, you can expect cloud to start forming at 1000ft.
The various quoted formula will give a reasonable estimate for cumulus clouds but stratus is a different animal.