The other day I tried this Espionage economy thing, on pangaea, standard, epic. I decreased my difficulty level to prince and chose Frederick, for his PHI/ORG traits (100% GP, cheaper courthouses). Built/chop rushed worker, worker, settler, warrior, GW, library and so on, and a couple of settlers in my second city.
I chose two tech-whores to invest my espionage points on (IIRC they were Darius and Pacal). Settled first GSpy, Built the SY with second, settled all the rest. Also built the ToA and popped a GM.
My plan was to produce huge amounts of melee troops in my non-capital cities. Still, I needed commerce and scientists to gain espionage points and beakers. But, my cities were in really bad locations. They didn't grow. They didn't produce much commerce, nor science. IIRC I had the slider at 60% research, 30% espionage, and didn't produce much gold, which I realized would be needed to upgrade my axes and swords to macemen as soon as I had stolen the techs from the AI. I did steal many techs from the AI, but had to research some of the most important techs (Civil Service, for instance) all by myself.
I left this game in the 800's, because things didn't go as planned. In fact, I realized that my plan itself had some flaws:
1) No specific plan regarding the source of espionage points and beakers. I didn't focus on my cities enough. I had some cottages, but started to build them rather late, having prioritized Code of laws and alphabet and having forgotten Pottery (I have played with SE for like 30 games prior to this and have forgotten how to cottage..
). Then, I didn't have enough scientists to boost my research, which I needed to hook up the techs the AI wasn't really eager to research.
2) No gold. I had no source of gold in the game, except the slider. I built the ToA for the GMerchant points (I know, sounds a little desperate...
)
3) Low difficulty level. The other civs were researchin stupid techs I didn't want and, all in all, played really badly.
I made up a solution to these problems. Firstly, in order to manage well with Esp economy, one should
1) cottage spam. One spy specialist per city in the early-mid game is enough for specialists. The slider can be set around 30% espionage, which will suffice with the Scotland Yard + increasing amount of Great spies in the capital.
2) found and spread a religion and build its shrine for easy gold. I think the gold is needed for this type of game. Because you basically won't have any techs that your opponents don't have - and therefore are not able to build more advanced troops than your opponents - you should build lots of primitive troops and when you finally steal those crucial techs like CS and Machinery, you should upgrade your army with the gold having gained from the shrine.
If you play with a civ that doesn't start with mysticism, you can still found confucianism fairly easily, by teching Meditation, Priesthood, Writing and CoL after masonry and some of the basic techs (Animal husbandry leads straight to Writing, but you'll have to research pottery after either fishing of agriculture). With meditation, you'll be able to build monasteries and spread Confucianism to your own cities and neighbours.
For the shrine you'll need a Great Prophet. You have presumably chopped the GW really early and then teched towards Code of laws. Priesthood is on the way so rush oracle for the GP points (Henge is not an option, you cannot have both GW and Henge). Alphabet is a good choise for the free tech. The first great person will almost surely be Spy, so settle it. The next one may be either Spy or Prophet (even chances), but you can increase the odds by building either a courthouse and assign a spy or building a temple and assigning a priest (Obelisk also allows a priest, and Madrassa allows two).
3) know which techs the AI prioritizes. You can invest your small amount of beakers to techs like paper-education-liberalism and steal philosophy, engineering and guilds. Avoid economics, take representation as the free tech after liberalism and research towards democracy for Security Bureaus, Statue of Liberty and emancipation. Steal Scientific method and rush for Communism.
The higher up in difficulty, the less you have to research yourself
These are my main thoughts. I played another game with Frederick (on Prince) and gained some success (haven't finished yet, its 1840 AD and my empire is looking pretty sovereign). Had Gandhi, Lincoln and Victoria as my neighbours, Gandhi grabbed London in the early game (WTH?! where have you gone, you Peaceful Gandhi...
), I stole huge amounts of techs from Gandhi and Linc. Spread Confu, vassalized Victoria with maces and trebs, later vassalized Linc with trebs, cavalry and rifles (upgraded CRII-CRIII-maces). During the 1700's I was running 30% esp, 60%sci, and started to steal techs (steel, steam power) from Sitting bull when Linc was about to collapse.
My typical Prince game months back was financed by SE, boosted by Pyramids and The Great Library. My capitol used to produce gold and great merchants Civics were Representation, Bureaucracy, Caste System, Decentralization and Theocracy. I used to build maces and trebs, beeline to rifling and destroy all my opponents with upgraded riflemen. I used to grab Assembly line by mid 1700's. So, compared to this, the Espionage economy doesn't seem to be effective enough. Maybe it's more effective on higher difficulty levels, when the AI will tech everything for you while you're building troops.