@Halcyan2 & ggmoyang,
Brilliant! Thanks for the replies. I am going to play out my current game (for training) and then give it another go.
Cheers!
Just beat it on Emperor at T92, moving on to Immortal, but a couple of notes.
Your horde should always be moving (and preferably fighting). If your horde isn't moving every turn, you're probably going to run out of time. In addition, every turn you don't have them attacking something is lost experience for your troops. Even if you don't want to take a city that turn (a situation I found myself in a few times where I was waiting for a city to raze down so my happiness would get back under control), find something else to fight. In those situations, I'd just beat the city I was working on down to nothing (but not take it), and leave 1-2 keshiks with logistics to keep it beat down (along with a horse to eventually take it), while the rest of my horde moved on to the next target.
By the end of the game, I had 9 elite Keshiks (with at least Logistics, 5 were level 9+ with 3 range and indirect fire, which made taking Arabia and Byzantium a walk in the park - Keshiks that can shoot over hills/forests from 3 hexes away are stupendously powerful), and 2 "cannon fodder" Keshiks (the poor sods on the front lines who spotted for my artillery Keshiks), though you need to take reasonably good care of them too (I think I lost 2-3 spotter Keshiks over the game). Most of the time, you can advance safely into unknown territory by just saving that one last movement point for a hasty retreat (i.e. never move that final hex into fog-of-war, especially near the front lines of any war), but every now and then (especially around Persia/India) the guy in front of the horde is going to get eaten by a Knight even when you take every precaution...so keep your most promoted Keshiks at the rear of your formation.
Beeline a second happiness policy after filling out honor (and, as others say above, the upgrade reduction cost policy is a really good first choice, lets you save your cash for other things, like rushing courthouses/colliseums).
Burn down any city you can that doesn't give you a unique luxury. It'll help manage happiness a lot - though don't be afraid to continue rampaging even when your empire is unhappy (first 40-50 turns I spent mostly unhappy, bouncing between -5 and -15) until I had my second happiness policy in place (I went Liberty/Meritocracy). Micromanaging cities is especially important - I didn't let any city except for my capitol go above 4 population (and even my capitol I kept at 5 until happiness wasn't a problem anymore) - any puppet that went higher than that was my next candidate for annexing/courthouse, whereupon I'd raze (or starve in the case of captured CS's or capitols) it back down to control unhappiness.
Prebuilding roads over ugly terrain is a great time saver, especially if you're going for India after the Chinas - I picked the Lhasa route, and I started building my road from the India side because it makes it easy for the horde to pour out of the pass and immediately take a city as a beachhead into India - if I had started from my side of Lhasa, they wouldn't have quite finished the last few hexes of the pass, and India's elephants would have made my life miserable as I tried to trickle out 1-2 keshiks each turn from the pass.
Have a plan - on the tight timeline in this scenario, especially with the distances involved, you should know where you're going next, preferably from the beginning of the game. For emperor, I went Uighur, Jin, W Xia (after Wu DOW), Wu, Lhasa, India, Persia, Arabia, Byzantium. It REALLY helped me having a save game from my "Khaaan" loss where I took the 100 turns fully exploring the entire map.
I actually missed an opportunity to take Cathy out right after Persia (I knew I didn't need her, as I had already decided to go Arabia then a short jaunt up to Byzantium) but she DOWd me right after I finished with Persia, forcing me to split my army to deal with her troops - since my troops in Arabia were doing just fine on their own, I could easily have taken advantage of the fact I had just liquidated her army and rolled her instead of Byzantium. It wouldn't have saved any time though, as my Arabia army was already knocking on Mecca's door when I finished her troops off, but it would have been fairly easy to knock off Russia and Byzantium simultaneously (given the tight isthmus Constantinople is on, I think 5 or so elite keshiks + a horse to take the city is all you really want there anyway) - that'll be my plan on Immortal, and while I'm doing that I'll see if I can put together a navy + amphib assault force in China without wrecking my economy or happiness (to see if I'm ready for the Deity challenge).
Anyway, good luck beating the scenario - it's a ton of fun once you get into the horde mentality and just go rampage.