After checking the German forum and testing in game, this seems to be the case:
For any normal coal mine (including Industrial Coal Mines), the production time is:
90 seconds if the deposit was found by an unskilled geologist
45 seconds if the deposit was found by the skill Quality Coal Mineshafts
30 seconds if the deposit was found by the Sooty Geologist
For an endless coal mine, the production time is:
60 seconds if the deposit was found by an unskilled geologist
30 seconds if the deposit was found by the skill Quality Coal Mineshafts
20 seconds if the deposit was found by the Sooty Geologist
There seems to be a bug that existing endless coal mines (which were found long before there was any Sooty) show as if they were found by Sooty, if you hover above them. However, the production time is still 30 seconds, not 20.
And no, there is no merit in giving the Sooty Geologist the skill Quality Coal Mineshafts. It's taken over by Sooty's own skill and they are not combined.