From the question "I see that many high-performance modules have heat sinks or heat dissipators on them."
Keep in mind that iMac's use SODIMM memory (often referred to as Laptop) for at least the last 10 years. The high performance modules you are referring to are desktop PC memory.
Also starting with 2012 models the 21" iMac is does not have a memory access plate. To not void your warranty you would need to take the 21" unit to an authorized service center or to apple to upgrade a 21" memory. The 27" iMac still has the access plate and is therefore user upgradable.