Well, you may be right that slavery is superior to caste production-wise... "production-wise" meaning upt, units per turn Especially shortterm, slavery clearly outweights caste, however, I think after some time caste will catch up... Consider, we'll most likely workshop new cities (ras, try to pillage towns/villages/... wherever possible and sensible), and they probably only get whipped once. AI cities are in good shape infrawise so we likely won't need to whip many buildings there anyway. And, Caste is very nice in new cities for running artist specs... we don't need to build any culture building at all unless the city is in danger of flipping, which "saves" us hammers we can spend on cossacks instead.
Does my memory serve me right, we are not able to change civics twice during a quick speed GA?