If playing as a charismatic leader you could set it up for 2 level 6 units/turn. 1 Altar city making priests, one horse city making horsemen. If the Empyrean holy city is in the Altar city then you'd need to tech up to priesthood for altars and not get evil civs automatically declaring on you. Still need to go to Religious Law to get theocracy in play, Warfare to get conquest and form of the titans (probably Military Strategy as well to get command posts if you don't have a Great Commander) and Way of the Wise, then Honour to get Empyrean. Still should be able to get the priest city up and running around the mid game.
Luckily after that the first city supercharges the run for the second city, which needs you to tech to warhorses. Luckily you should already have Horseback riding in order to get, and probably teched to Bronze working in an effort to survive the early game. With the Empyrean holy shrine you'll have Sun and Spirit mana, so make a quick run to divination then build a Law and Mind node. Then the tower to get Warhorses for free. While that is going on tech Smelting and Iron Working. Send shade engineers to the second city to bolster its production to crank out ride of the nine in a short time.
Its hardly a foolproof strategy since it relies on founding the empyrean, building form of the titan, generating 5 great prophets and 2 great commanders(or being organized) and finally the ride of the nine kings. While you're doing all this you'll be pretty fragile, a main reason why it is better to go for empyrean shrine rather than have the auto war declarations that come with the fifth altar. Although it does also mean that in an emergency your best two cities can crank out a super elite garrison force. Setting up a shade assembly is quite fun, but basically any type of victory is a cake walk from there as your technology gradually dwarfs the opponents.
The easiest victory to go for after the assembly line completion is probably cultural. Adding two great artists per turn to your cities adds up fast.
Well, in order to build Altar pieces you need to generate 6 great priests, and to generate great priest you need to assign a lot of priest specialist. And since they are no better then priest specialists from other civs, you're not taking benefit from better specialist Sidar have.
It's worth noting though that the altar itself ends up supercharging those priest specialists beyond what the Sidar bonuses do. An altar V great prophet will be adding 4 hammers and 5 gold. An altar VII one will be adding 5 hammers. Altar boosted priests and great prophets aren't exactly weak specialists.
