I disagree with the statement that they give u an extra 10%.
I agree with the thing about diet, as much as diet is a big part of it I don't think it's as big as people claim. I think sleep is a major piece being left out.
On the roid side I don't think they give you "an extra 10%", I think they magnify the work you do, '0 work' magnified to any degree is still 0 gains.
They allow you to train harder and longer, with more stamina and strength, and they recover you faster for more workouts per week and more reps per workout. I think in a practical sense they allow you to pack a year of lifting into a few months, and give you a performance boost right away that the year of natty lifting would not give. The agression and motivation in the gym is also not to be overlooked.