I stopped short from finding that out in my analysis of the espionage system because of the notice that it will get overhauled in 2.0

Common thought though is that it allows that much more % of spending to be allowed to train spies.  So the Iconians with X income set at Y% spendin towards spies actually will spend double than a race with no espionage bonuses with the same income and % due to their +100 espionage.

Rumors also surrounded their possibilty to gain intel level faster, but as I said, I stopped short of testing that

To see what I do have now (and it will get updated once 2.0 comes out and I get ot play with it again) check this post Espionage Intel Levels