Just an update on this, or lack of update. (Carrier Infinity supply air temp goes up from low to high stage, suction pressure goes up - good performance at low stage) Original thread:
http://hvac-talk.com/vbb/showthread.php?t=1113821

My original installer (from 2007) is waiting on a response from the local Carrier technical representative. In the meantime, I've been taking careful return/supply air temps at the air handler (return duct at air handler and a few feet downstream of evaporator). I've taken them almost every day, early morning and middle of these hot afternoons and the numbers are pretty consistent. BTU performance from low to high stage is essentially constant. This morning:

LOW STAGE:
Return Dry Bulb: 71.8
Return Wet Bulb: 58.7
Supply Dry Bulb: 48.7
Supply Wet Bulb:47.4
Following 3 readings from Infinity Controller
CFM: 882
Blower RPM: 599
Static Pressure: .24


HIGH STAGE:
Return Dry Bulb: 71.5
Return Web Bulb: 59
Supply Dry Bulb: 55.4
Supply Wet Bulb: 52.7
Following 3 readings from Infinity Controller
CFM: 1400
Blower RPM: 960
Static Pressure: .61