I played another Shaka opening, and I change my mind about the Impi. It's a solid UU, the nasty sucker is on par with immortals thanks to cover and mobility which are available to it with a building you'll be getting anyway.
Edit: I'm in 1712 AD on emperor pulling 1300 + bpt. I have about 40% of the world, 35% of which I took solely with impi hit and run war declarations. Map is fractal, wound up with 3 continents:
Me, persia, france on one.
Asoka and Vikings in middle.
Germany and Eithiopia on far west continent.
The fun thing about the impi/ikhanda combo is the war declare/cease fire city sniping. Rushed and killed persia NP (no way they were hooking up any resources). I didn't bombard anything, so war weariness was terrible while fighting France, but it didn't matter. Declare war, steal like 1-3 workers that turn if possible, and either take a city on turn 1 or turn 2. This is pretty easy with 12 impi, and they can (in godly "full monty" numbers) take on longbows even. It didn't matter that France had iron. I'd declare war, take a city or two, then take peace. This was hard to pay for so I cottaged everything. New cities got ikhanda pop rushed, workers would chop out more impi while in peace, then I'd go again. I'd use capture money for deficit research when possible to keep up (wound up lucking out on the lib race around 1300 AD).
The trick with impi is to take a city or two before they can react, then camp out until peace. Rinse and repeat. It's actually not that hard to do because they are very, very cheap to produce, especially as time goes on. If you want to keep making them after engineering trade away iron for a bit

. You think you'd take diplo hits for doing this and you will, but if you target religious scape goats or just take a side then some people will either like or love you even if you declare war on their enemy 15 times

. This tactic is great for wearing a civ down while assimilating cities.
Later on barracks + ikhanda add up to -70% maintenance. Everyone talks about how great the HRE UB is once he has it up, and this is very close to that with only slightly extra investment (aka cities that you wouldn't normally put a barracks in, you do so).
This strategy relies on LOTS of workers. Expansive has synergy there, but stealing them is still cheaper if you can manage, which you probably should be able to

. Shaka is definitely a top leader in my eyes due to his early game dominance in non isolation (where he still isn't horrible, just not great). Still undecided on Ragnar vs KK.