An apparatus comprising: a compressor having a suction port, a discharge port, and an intermediate port; a condenser; an evaporator; an economizer heat exchanger; a conduit system: coupling the condenser to the discharge port; coupling the economizer heat exchanger to the condenser; cooperating with the economizer heat exchanger and the evaporator to define a first flow path between the economizer heat exchanger and the suction port; cooperating with the economizer heat exchanger to definine a second flow path between the economizer heat exchanger and the intermediate port, bypassing the evaporator; having one or more valves for selectively blocking and unblocking the second flow path; and a control system: coupled to the one or more valves and configured to alternately operate the apparatus in a plurality of modes including: a standard mode essentially wherein a refrigerant flow from the condenser passes along the first flowpath and not the second flowpath; and an economized mode essentially wherein a refrigerant flow splits into: a first portion passing along the first flow path; and a second portion extending through the second flow path section to return to the intermediate port; and characterized in that the control system is configured to shift the apparatus between the modes responsive to a determined efficiency reflecting a combination of at least two of: compressor isentropic efficiency; condenser efficiency; evaporator efficiency; efficiency of hardware mechanically powering the compressor; and a mode-associated cycling efficiency. a compressor having a suction port, a discharge port, and an intermediate port; a condenser; an evaporator; an economizer heat exchanger; a conduit system: coupling the condenser to the discharge port; coupling the economizer heat exchanger to the condenser; cooperating with the economizer heat exchanger and the evaporator to define a first flow path between the economizer heat exchanger and the suction port; cooperating with the economizer heat exchanger to definine a second flow path between the economizer heat exchanger and the intermediate port, bypassing the evaporator; having one or more valves for selectively blocking and unblocking the second flow path; and coupling the condenser to the discharge port; coupling the economizer heat exchanger to the condenser; cooperating with the economizer heat exchanger and the evaporator to define a first flow path between the economizer heat exchanger and the suction port; cooperating with the economizer heat exchanger to definine a second flow path between the economizer heat exchanger and the intermediate port, bypassing the evaporator; having one or more valves for selectively blocking and unblocking the second flow path; and a control system: coupled to the one or more valves and configured to alternately operate the apparatus in a plurality of modes including: a standard mode essentially wherein a refrigerant flow from the condenser passes along the first flowpath and not the second flowpath; and an economized mode essentially wherein a refrigerant flow splits into: a first portion passing along the first flow path; and a second portion extending through the second flow path section to return to the intermediate port; and coupled to the one or more valves and configured to alternately operate the apparatus in a plurality of modes including: a standard mode essentially wherein a refrigerant flow from the condenser passes along the first flowpath and not the second flowpath; and an economized mode essentially wherein a refrigerant flow splits into: a first portion passing along the first flow path; and a second portion extending through the second flow path section to return to the intermediate port; and a standard mode essentially wherein a refrigerant flow from the condenser passes along the first flowpath and not the second flowpath; and an economized mode essentially wherein a refrigerant flow splits into: a first portion passing along the first flow path; and a second portion extending through the second flow path section to return to the intermediate port; and characterized in that the control system is configured to shift the apparatus between the modes responsive to a determined efficiency reflecting a combination of at least two of: compressor isentropic efficiency; condenser efficiency; evaporator efficiency; efficiency of hardware mechanically powering the compressor; and a mode-associated cycling efficiency.