Espionage-spending past advanced has 2 effects:
1) Information collected on the "Stats" page does not automatically refresh. Continued spending is necessary to update treasury figures, %-bonuses and current techs being researched.
This is important if you are trying to track such things as who:
- recently built/improved a SB;
- has better combat-bonuses than you;
- is "benefitting" from the Draginol event, etc.
2) Destabilization.
Destabilization is poorly documented, and as a result, not too well understood (or employed). Basically, destabilization lowers Morale, which (in turn) affects planetary population growth, sustainable tax rates, and the necessity to spend money on morale/resistance-bonus buildings and propaganda rather than going straight to Military.
Destabilization will eventually result in Morales low enough that secession is triggered. But this is very long-term, requires maintenance of a continuous level of spending, and ensures a near-permanent state of war.
To benefit directly from destabilization, you must obtain cultural-superiority in the sector any planet secedes in. If so, it will flip to you; otherwise, it will join the I-League, or whoever else is culturally-dominant (if the I-League is at war with the target AI).
The effect of destablization spending is affected by:
- relative population size of the target AI; and
- your espionage rating vs. your opponent's.
I have seen techs stolen, but only by Minors (!). In one game, a cut-scene of a Scout heading towards a planet appeared, with the message that the Minor had "...stolen all X's technology and was willing to trade it in exchange for Y" - in this case, a planet they could relocate to. In this manner, I traded for Dreadnought technology without having researched any prerequisites.
However, I myse;f have never seen my operatives acquire a tech, even with espionage spending maximized over multiple turns.
~SDC~