CCO is not a handover : resource is not reserved beforehand, network doesn’t know that mobile is changing cell, and mobile has to do a standard (full) setup on the target cell.
It is possible to perform a real PS HO from 3G to 2G, involving a procedure which is specified in 3GPP
as “IRAT HO”. Alternatively it is also possible to use the CCO to perform mobility from 3G to 2G. Both procedures exist, and the operator has to choose whether or not to activate PS HO.
Activating PS HO will (mostly) replace the CCO.
Most operators I know do not activate PS HO, in order to avoid compressed mode during PS transfer. They’d rather let the PS transfer continue until the drop. Since it is “bursty”, there are very high chances that the transfer will be over before the drop.
So yeah, back to the point, you can define the KPI for mobility PS 3G to 2G based on CCO and/or PS HO, whichever one is used.