Well you certainly know your spies.
Science and siphon fund actually is much better than steal a Great work for you don't need to move to get all
Well its more complicated than you think. The mechanics are straight forward but the strategy is not.
A runaway culture civ relies heavily on great works so its a good way to knock them down. You can take the works yourself..... or sell them to others and they are worth a lot of money now. You can spread the culture growth around. This has many knock on effects. You can also place works in non steal able places like the bank or Apadna. I have noticed the AI unfortunately does this also. Also siphon and stealing funds are often easier to get caught doing. The AI tends to place their spies there and they cannot protect all theaters. A civ has cities close to each other so its an additional few turns between them. Disguise is a good promotion for an art thief.
Two success lead to master spy(level 4) and can steal a tech with 75% without gaining soure. Won't it be more effective compared to Ace driver?
Death is the driving factor here. Have a super fast spy but with each mission your chance of getting killed increases. Ace driver ... 4 levels higher?... that's like super spy twice as good and more importantly a fast getaway means more stealing afterward. More to the point... if you are settling yourself i one city as you say for a 'long time' what would be better?... linguist and Disguise or Linguist and Ace driver? Or as you say.. you would take Con artist and Disguise..... I have no issue with that ... but I would take Technologist and ace driver as the chance of capture is higher.
The point is you have more than 1 spy and regardless some will get caught and you do need to spread the skills around. I am just glad now France can start with two with 2 promotions each which means you can have a Quartermaster at home and a spy abroad. That extra level helps. As France I will build a BIG city with a campus and CH next to each just for my quatermaster to get to Level 4. Selling spies back is also good trade and they seem not to have a memory of how good she is at capturing spies.
Promotion list for those unaware
Ace Driver: If caught on a mission, have a much higher chance of escape (+4 levels).
Cat Burglar: Steal Great Works as if 2 levels more experienced.
Con Artist: Siphon Funds as if 2 levels more experienced.
Demolitions: Sabotage Production as if 2 levels more experienced.
Disguise: Takes no time to establish presence in an enemy city.
Guerilla Leader: Recruit Partisans as if 2 levels more experienced.
Linguist: Time to complete all missions reduced by 25%.
Quartermaster: If this Spy is in home territory, all your Spies operate at +1 level.
Rocket Scientist: Disrupt Rocketry as if 2 levels more experienced.
Seduction: Counterspy as if 2 levels more experienced.
Technologist: Steal Technology as if 2 levels more experienced