The reason why it does not help very much is due to the fact it gets added together with fire/light mastery.
So, instead of having forexample: +550% to light damage, you get +570% to light damage. The change is small. 4,5% to be excact in this scenario, actually its smaller because i think fire/light mastery adds together with synergies.
So lets say you had +500% from synergies, and +550% from light mastery.
Then with a 20% eshuta that would be:
1070%.
And the difference between 1050% and 1070% is: 100-(1050/1070*100) = 1,89%
So ur base lightning damage(without synergies and without mastery) would gain a 1,89% boost from the 20% eshuta.