It's good in the early stage after founding or enhancing, but decrease much in later eras. While Iam able to generate over 2k culture in modern era with a 5 city Morocco, my faith output is around 220. Transformed, that's an amount of 55 culture, an increase of 2.5% in total. Compare it to mosque with 20% increase.
If you split that culture to 5 cities, it's 11 culture +4 more yields. More valuable yields and more of them than other follower beliefs, but it needs longer to reach this values. I think it's good especially for 4-5 city empires, Byzantium and in combination with ancestor worship, but not overpowered in general. The yields per city will decrease with the amount of cities you will have. Looks fine for me.
I agree. Right now the belief is an early game strength, while mosques are your late game big boost. Seems a reasonable balance to me.