It's not really fair to compare the eureka Great Scientists to Newton or Einstein, which are both far too powerful. Let's instead compare two renaissance great scientists, Emilie Du Chatelet and Galileo Galilei. Du Chatelet triggers 3 eurekas from the Renaissance or Industrial Era, and Galileo gives 250 science for each adj mountain tile. Standard Speed.
Du Chatelet: Assuming you trigger one late renaissance eureka (40% of 660 science) one early industrial eureka (40% of 700 science) and one late industrial era tech (40% of 805 science) you get a total of 866 science. This is a good but not ideal scenario for this great scientist.
Galileo: 3 mountains is pretty standard, 4 is pretty great. This comes out to 750-1000 science.
Seems pretty balanced to me.