I hadn't thought of when you had +100 prestige and the trade centre policy, though even if you have both (which is a big if for a majority of players, especially since disinheriting an heir can often be so useful) it only reduces the time it takes for your AE to cool down by about 8% overall if you take the -20% AE instead of the +25% IR. If you took Humanist Ideas already the -20% AE is much better, but otherwise while I will agree that the AE reduction is more useful in some instances it hardly makes for something splendidly amazing to sell the whole idea group.
1/0,8 = 1,25
Even in the worst case, the AE reduction in espionage is as good as diplomatic 25% IR.
1/0,7 = 1,43
Add 100% spy network to to mix for another 18% extra AE capacity
1/0,6 = 1,67
Add age ability, national ideas or religion for another 24% extra
1/0,55 = 1,82
Now add bonus from 50 prestige (which is very manageable even with monarchies).
1/0,35 = 2,86
When you get lucky with curia controller, you get so little AE it cools off before you have finished your next war (or whilst waiting out truces). Granted curia controller is rng dependent, but to me it happens once every run.
I did the calculations last year on it (I love taking Espionage ideas) and while I don't have them saved anymore what I do remember is that with just +50% spy network construction and no other modifiers you will typically sit on somewhere between 63 to 100 network with the nation you are targeting (unless they are your rival). You will also typically be on 100 for about four months on average before being discovered and heading back down to 63 again. This isn't too shabby at all, but it still only means an average of -8% AE reduction which only applies to that single nation you are targeting (which generally isn't too helpful in stopping coalitions).
In most wars, you take the majority of land from a single nation. That land will grant 10% less AE. It does not decrease AE-opinion of that nation, but reduces AE from all land taken from that nation.
Also, do note you can swap your merchant policy for another +25% spy network construction to temporarily boost your spies, until you can peace out at max network.
I had misunderstood what you wrote, and it does sound like a cool strategy, but it is also quite niche and if we are going for min-maxing on how many points we spend conquering provinces than Influence wins out, and Influence + Administrative absolutely crushes the playing field (sort of cheating since I am talking about two idea groups, but I mention it since there is nothing Espionage combines with that really compares currently).
Overall, Espionage is alright but struggles to compete against and finds its place among Diplo or Influence (or Trade or Exploration).
Influence decreases diplo points for peace treaties by 50%. If you use this trick, you will have claims on all land you wish to take and reduce the cost to 0. It takes more effort, but is twice as good.
You should try to play a game as a free city. You can conquer half of europe whilst still owning only 1 province yourself.
Recommended idea groups: Espionage, Influence, Economic.
Since you don't directly core provinces or annex vassals, you don't pay any MP. This allows you to develop lands instead, which will keep your vassals perpetually loyal.
In this scenario, AE is the only ''expansion tax'' you pay, so reducing it leads to rather comical situations.