Well if you went FoL, I usually just go up to priesthood for blooming priests, not really to use them much in offensive manners, though tigers can be good fodders. So the way I use them doesn't really justify the necessity to swap to a full 100 turns of spiritual.
I find that a mounted/metal approach works well, picking up copper for your chargers to make them 6/4, and str 5 axemen for some defensive units in your stack. (though basic centuars do get defensive bonuses and are decent stack defenders pre-copper axemen)
I suppose an mounted/archery approach would be very strong defensively, and you'd need to pick up archery anyway for stirrups. But I do find that having at least copper for your chargers is pretty important for you to have a good offensive punch midgame. (By the time you need archery for stirrups, since you already hit trade, you usually could find someone to trade for it if you went mounted/metal--usually Elohim has it

)
I'm somewhat reserved about a mounted/mage approach mainly due to a couple of factors.
Usually focusing on the sorcery line you'd want to do it pretty early, to churn out enough adepts to sit around so that they'll mature to hit mages once you hit sorcery. That could possibly delay your mounted rush, and as I mentioned in my earlier post, I find that an early mounted rush is a somewhat strong suit for the kurios, hopefully before your rival picks up copper (or that he hasn't built too many copper weaponed warriors before you pillaged his mine).
Same like archery as a secondary line, you'd still want to hit copper for some decent offensive chargers, thus more delay somewhat.
They'll be also lacking defensive units around midgame, when the defensive strength of your centuars become obselete.
I do certainly believe that the mounted/sorcery approach does have some interesting possibilities. Don't get me wrong because I certainly think its worthwhile to invest in KoTE, just that swapping over to arcane and trying to tech up to sorcery early would mean a huge delay in building up your army.
However, just picking up KoTE is very useful for the kurios, for utility adepts. Their palace mana are sun, spirit, water. Spirit adepts are very useful for courage for healing in enemy territory. Water lets you get rid of unwanted deserts to plains, letting you settle in those floodplain rich areas with a few desert tiles mixed in. And supposing you founded FoL and built the Song, Nature is good for treetop defense, making it very difficult to even dislodge your str 4 centuars in enemy forests.
However, I do understand that the strategy of going FoL + Mounted/Metal do mean that you basically forgo both your religious heroes. Your core army would consist of centuars/chargers/axemen early on, and chargers/centuar archers later on, before you get that huge leap to centuar guards. But by the time you get guards which are promoted from your high level chargers, which are 14/11 with iron, you should be able to cut down most units in the field that you don't really need those heroes. They should pick up cover II as well so they could even destroy most garrisoned city defenders save maybe crossbowmen.
But yea, I sometimes do miss Yvain

He can be such a beast with enough nature nodes. (Kyriel on the other hand is such an underpowered hero for such a late tech) Though should you decide to go down the recon/spiritual line instead to pick up both heroes, swap over to a religion like OO to get back to neutral before going to FoL, so that you could benefit from druids while you hoard nature nodes to pump up their nature affinity str.