Correct, most people would be better off with a simple 40mm 5V fan blowing air over the thing. If you want something passive it's going to need to be at least the size of a Northbridge heat sink. Much bigger than what most people use.

I don't think you need shims. All the cores are in the large die. My stock heatsink had plastic over the thermal pad which should have been removed. I discovered it while in the process of replacing the heatsink so i didn't do any testing with it removed.

I've attached this heatsink with thermal paste and a noctua fan and have had a decent improvement in temps.

Using this as a benchmark, my stock times in time2throttle were:

2 instances- 42.4 s

3 - 6.9

4 - 2.3

After the upgrade the times were:

2 - never hit

3 - 46

4 - 5

Suffice to say, I believe the heatsink is making adequate contact without any shimming.

note: time2throttle only runs properly in ubuntu.