A 4nm chip Qualcomm SM8550-AB Snapdragon 8 Gen 2 must be more energy-efficient than a 7nm chip Qualcomm SM8150 Snapdragon 855. Its peak power consumption could be higher, but it must have more modern power saving and idle states.
What battery life change have you noticed? Not significant but how much? 5% lower, 20% lower? 10% higher?
My guess is that my new device is plagued with bloatware which sucks the life out of my battery.
It is not as simple as a new chip vs old chip. The screen could be worse, there could be other chips on the phone/device that are sucking up power, a bad modem could cause the phone to try to find the cell network more often or less efficiently, etc, etc. Bloat ware is definitely a possibility. Even the manufacturer could have enabled or disabled different modes on the chip and firmware for power or performance reasons and these companies don't always get it right.
It's why people still need to read phone reviews and not just rely on the list of specs.
Regarding my Pixel, the difference is probably below 10% as I don't notice it usually. The battery life is less than it was brand-new but ALL batteries show degradation behavior over time used and I have been bad about discharging the battery completely several times which is not ideal battery charging parameters. The amount of apps has also steadily grown over time and my storage is now completely full. So the phone is probably spending more time moving data around on NAND or not storing new data ideally in blank space NAND, and there are more apps phoning home and waking the phone up for updates or to push data in and out.
In my case, the OS version has had no practical effect and I attribute all of the change to the battery physically aging and being used/abused by me as well as having more software being used. In sleep mode (screen is off, phone is not touched and not charging), the battery life seems identical to how it was in launch.