If I had no interested in keeping Isfahan myself, I would not care if my vassal taps the Mosque or not. I'd notice the difference in Persian vassals' taxes at some point in future, when it no longer matters, because nothing does. I would make sure though, that the holder is of my dynasty, preferably my childer and is strong counselor material worthy enough to pass rich kingdom to, at some point.

I avoid switching the Road of zealous characters. I mean it's easy to lose the trait mechanically, but it still feels like they, the character. would strongly disapprove. But, if I had muslim wonders on my hands and my willpower is not enough to contain the Beast of minimaxing, I would consider adding islamic syncretism tenet to my faith.

I would save too, just in case. It worked before, basically treating your faith as Islamic for all intents and purposes, but it does not mean it still works in the current build. Also, for many decisions the list of eligible faiths is carved in stone. So sometimes when you make a "new" faith, even if you change nothing but the name, you still lose access.

If it's not clear, the above is an answer to what I would do, not to ever persistent why do YOU obey.

My thinking was the development growth provided by an activated Friday Mosque which in time helps all counties around Isfahan. So in time makes the entire realm stronger even I do not personally hold Isfahan.

Also.. ya.. I have found I am a bit ODC when it comes to special buildings and CKs 3. It really bothers me them just sitting their doing nothing deactivated when it probably should not so much.

Very good point of not switching the faith of zealous characters for role playing reasons.

All I was asking for in my OP was what others would do. :) Thank you for your response. :)

I understood your line of thoughts, but I believe you overestimate impact of development, especially when it's filtered through vassals. Let's take a duke with 4 gold of income, giving you 0.4 gold in normal taxes. Whole duke's domain increases it's development by 1 level. His income is now 4.02, giving you whopping 0.002 extra taxes. Yay?

It will take about 30 years for mosque to yield that extra level. And it won't raise all Persia evenly. Development lag is huge. When Rome is capital and I hold both Spoletto and Ancona, lag is 10 levels of development in every link. I.e. 31 in Spoletto, 21 in Ancona when Rome is already 41. 51-41-31, 61-51-41. And that's when I control whole chain directly, without NPC vassal in between to collect as many development maluses as possible.

When development reaches point when you can upgrade your own ducal holding, that's indeed a significant boost. But it makes your idea about trading Isfahan(20) for Shiraz (18) all the more criminal. It would be terrible even if they were equally developed. Isfahan sucks as capital hub, hands down. The only good thing about it, Jamal already holds it with 100 control.

EDIT: Math.