Hey Simon,
Actually, that's the other way around :-> SimConnect clients built with the SP2 version of the FSX SDK will also work with ESP v1 (it should actually support all previous versions, but I've only tried it with SP2 SDK built clients).
SimConnect clients built with the ESP SDK will only talk to ESP (FSX isn't foreward compatible with versions that weren't released yet when it was :-> ). I suppose FS11 and/or TS2 might have recognized ESP v1 clients, but that's a moot point now :-<
Tim