I think (and Tom can probably correct me on this!) that the HA RENIX CPS has the sensor tip "advanced" two or three degrees to start the burn earlier (and I may have this reversed at the moment...) which is supposed to help compensate for the paucity of air at altitude.
I've driven RENIX XJ's across the country (back and forth from SJC to IND, and from SJC to ATL once) and didn't have any trouble at all going through the Rockies, the Sierras, or through Denver - so the utility of the thing is a bit suspect to me anyhow. However, I'm sure there are those that note it - and this would probably be the only real "failing" of the RENIX system (in my mind...) if it can't effectively and consistently correct for altitude with the MAP sensor and IAT giving air density readings anyhow.
Still others have reported a "butt dyno" increase in power from using the HA CPS at MSL, due to the peak burn/combustion pressure being closer to the peak leverage of the crankshaft/connecting rod assembly (12* ATDC, as I recall.) Again, this isn't something I'm worried about, so I've not dug into it. And, since I've not got an engine set up on a dyno, I can't quantify this effectively anyhow.