I'd be keen to know this myself. AFAIK, Karthadast doesn't actually need to sustain the Romani in order to get its reform, but I could be wrong; and apart from building up MICs I think the reform comes about automatically in 200BC.
No idea what it unlocks, but one would presume Elite African Pikemen at least, and perhaps also Elite African Infantry, Sacred Band Infantry and the Heavy versions of Libyan and Liby-Phoenician Spears/Infantry.