Tasunke is basically overpowered to hell at this point. We'll see where it goes in the future.
I didn't use Tasunke. Yes, his promotions are nice, but a bit redundant, really. You already have the best horsemen in the game. Further enhancements are nice, but not necessary (especally since you'll almost certainly be getting Ride of the Nine Kings), and raiding only works when you have something worth pillaging.
Rhoanna, on the other hand, has the most powerful trait from Vanilla, financial, which can turn her cities into cash machines. This allows her to take advantage of another one of the Hippus' chief advantages: horse mercenaries. Granted, they aren't the most powerful unit in the game, but when there's fifty of them, and reinforcements are available instantly in every city you own, they don't have to be. Plus, you can always enchant their weapons.
I stuck with Runes as my religion, but I was sure to found the Undercouncil. Why? SLAVE TRADE. Now, you can use that money on a disposable armies and a disposable workforce. Your treasury can now be both your sword and your tools, and you can build or conquer your way to world domination however you please.