Yes, its that "disc". I always considered the disc on top of the CPU as the heat sink and on my 3 Wallstreets there is no heat/thermal compound goo or anything else on it or the CPU. Where that disc is pressed against the CPU it is polished to promote the heat transfer to its maximum level. (That is why I consider it as the heat sink.)Just so we are on the same page, you are talking about removing the black disc, right? Black disk being the heat spreader held against the CPU die by a square plastic piece around the whole CPU package. Then the big metal plate under the keyboard is the heatsink.
On the underside of the black disc I forget what Apple used most commonly, and I would check mine but I am quite certain I squirted some thermal compound under the heatspreaders of my daughtercards after cleaning them out however many years ago. Normally I would want to refer to the disk as an IHS(integrated heat spreader, like what I removed from my i7 4770k!) but it is not exactly integrated, just more slapped on IMO.
Oh, and my mentioning the TL iMac was incorrect, I forgot about it's rectangular heatsink that butts up directly against the die and is held down by a clip to whatever that daughtercard cage thingy is.
The metal plate that is part of the em-shield under the keyboard that you refer to as the heat sink, I see that there is some kind of thermal/insulating tape there. I see now that it it part of the heat dissipating mechanism; similar to old CB Radios where the final amplifying transistors are on a heat sink and then the heat sinks up against the radio chassis. In those cases, the radio chassis is not considered as the heat sink but as electronic ground. It draws away the heat from the finals was a secondary function being exploited. This design is also in the LC and IIcx/ci Power Supplies, where there are power transistors/regulators in the middle of the board tied to a heat sink and then that heat sink tied the PSU's case. The PSU casing is not the heat sink but it being taken advantaged of as such for this process.
In both cases there is some kind of heat/thermal compound goo on the heat sink and the chassis/psu casing. So I also believe this piece on the Wallstreet is doing the same thing - doing one job of EM shielding while at the same time being taken advantage of for drawing away heat. In the past in testing, I have ran the Wallstreet (a 233MHz) for three hours as a test without the shielding but with just the heat sink disc without ill effect on the system.