Well, ESP didn't have a SP1, but it does have a different SimConnect version number from FSX (which has 3 different version numbers of its own for RTM, SP1, & SP2) - but ESP also allows connections from clients built using the FSX-SP2 version of SimConnect (it might also allow RTM & SP1, I haven't tried those), so you can use FSX-SP2 as a happy medium to support both FSX-SP2/XPack & ESP v1 targets.
An aircraft that works in FSX-SP2 (be it FSX Native or FS9 PortOver) will also work in ESP v1 (although its possible that ESP might not include all the old gauges that a FS9 aircraft might reference, so there could be holes in the panel).
Tim