Processor Upgrade Experiment Results
Knowing how the processor is mounted in a particular Mac only is half the battle. Determining whether or not a processor upgrade will function originally required experimentation by brave souls.
HardMac was able to successfully upgrade an iMac "Core i5" 2.66 27" (Late 2009) -- equipped by default with a 2.66 GHz "Core i5" I5-750 processor -- to a 2.93 GHz "Core i7" I7-870 processor. It is worth noting that this model also could be equipped with a 2.8 GHz "Core i7" I7-860 processor at the time of purchase via build-to-order configuration.
Although no details are provided, multiple third-party forum readers also reported that swapping the stock 3.2 GHz "Core i3" I3-550 processor in the iMac "Core i3" 3.2 27" (Mid-2010) with a 2.93 GHz "Core i7" I7-870 processor worked without a problem. By default, this model was available via build-to-order with a faster 3.6 GHz "Core i5" I5-680 processor.
However, there are limits to the processor upgrade options, as well. In response to a request for reader input, Jason Riley of Sydney, Australia kindly shared the results of his efforts to upgrade a iMac "Core i3" 3.06 21.5" (Mid-2010) with an LGA 1156-socketed 2.93 GHz "Core i7" (I7-870) processor. He discovered:
The new processor physically fits fine but the computer wont boot with the new chip. I tried swapping the CPUs back and forth a couple of times just in case I missed anything but each time the i7 CPU failed to boot.
I suggest there are one of three possibilities:
Apple have restricted the logicboard firmware to specific CPUs.
There is not enough power for the i7 (95W as opposed to 74W).
The logicboard requires the graphics capabilities of the FCLGA 1156 Socket chips.
Additional readers -- in particular the very helpful Andy Hucko of Bratislava, Slovakia -- were able to confirm that there are not logicboard firmware restrictions, and the above upgrade was restricted only by power, or specifically, the TDP (Thermal Design Power), which refers to the heat dissipation capability of a particular CPU cooling system.
Andy was able to successfully upgrade the entry-level iMac "Core i3" 3.06 21.5" (Mid-2010) with a 3.6 GHz "Core i5" (I5-680) processor, which is the fastest available CPU with the same TDP as the original processor in this model.
Processor Upgrade Summary
From the above experiments, one can safely conclude that thankfully there are not restrictions on processor upgrades in firmware for any of the Aluminum iMac models.
However, one only can upgrade the processor in an Aluminum iMac with a socket-compatible processor that has the same, or lower, TDP as the originally installed processor.