Flex uses by standard RPC/AMF to communicate with the server side.
It’s fast, supported typed data, etc, etc …
Even having been created many years ago and it’s not the standard seems that it’s still the best protocol for communication however seems that it’s not completed support by Haxe.
It’s that true today or the only way to port a Flex application to Haxe is by using REST/JSON ?
If so, I would say that the very fist step would be to convert RPC/AMF to REST/JSON inside Flex before even start to port AS3 and MXML.