Due to the physical length of the previous log, I have written this one for a more detailed analysis and to posit a few ideas to keep performance along around longer despite the thermal limitations.

Power vs Performance

The smoothed, peak power draw from the i7 is 113 W. For a 65 W TDP part. *IF* you equate a 1:1 power vs performance scaling between the i5 & i7, that increased consumption is equivalent to ~11 cores. Granted this lasts for about 2 seconds, but still. Hyperthreading/SMT gives an advantage of about 25% per logical core when the physical core is fully loaded. It can be higher or lower depending on the actual type of computations, and even lower than if only a single thread occupied the core.

Apple knows this, and they have generally, at least from an end-user perspective, mitigated perceptive interruptions as apart of the user experience. In terms of speed, Mac OS X 10.6.8 is still a favorite of mine. Newer macOS versions have improvements and their fair share of regressions too. One of which is that they know which cores are physical, and which are virtual. First and foremost, Apple's scheduler prioritizes the utilization of the physical cores first and then starts adding things to the virtual ones once the system in inundated with heavier workloads. The problem though is that with more power comes more heat.

Improvements via TIM

So early on, prior to ~3-4 minutes, the cooling solution is able to "cope" with the heat and there is a delay before it begins to submit to the heat. The MX-2 holds the later time due to better properties, thermal conductivity and/or bond line thickness, but it only goes so far. I bet that even if I used a liquid metal TIM, the same effect will still occur, though I'm not sure if it'll happen sooner due to the increased heat flux, or later from the greater transparency of the TIM.

That's a rabbit hole I won't bother trying to address, though I can say that better TIM will increase the performance of the solution under short, pulsed, or transient workloads. Everything significantly short of Jeremy Clarkson yelling "moar power" and the TIM will help, but it does not fix the inadequacy of Apple's cooling solution.  

Performance vs Physics

Once you pass about 10 seconds of loading, it doesn't really matter what TIM you've got because the cooling solution's design is inadequate. Because I loaded up all physical and virtual cores, much more heat was generated than normal. Hotter cores means that you'll reach the limiting temperature sooner rather than later. Even running a short benchmark like Cinebench R15 nets a MC:SC ratio speedup of 7:1. Yeah, those 6 extra virtual cores net a grand total of 1 more core of performance on the i7-8700B when combined with Apple's cooling solution. FOR $200!

Anyhow, in order to get the best performance in the cooling solution as is, you need to limits the number of threads that will load the CPU. If you try and load everything, the heat will swamp it out and limit individual cores. Due to the reduced speed of the i5-8500B, you don't have a frequency drop when that 6th core is loaded. However, for sustained loading with the i7-8700B, 5 threads is where diminishing returns begin happening as we loose ~2.8% performance and when 6 threads are active, you'll loose 6.5% performance due to the frequency drop from increased thermals and throttling. If you load it up to the full 12 threads, you'll top out at the equivalent of 8 cores but running slow enough to match 7 cores and suffer a 12.5% performance penalty. That means Hyperthreading/SMT nets a 33.3% increase at best on this architecture, at least by power/thermal loads.

To reiterate, these performance hits do not occur on the i5-8500B. Following others' suggestions and recommendations, buying this CPU is a sweet spot in performance if you're just going to run the computer in its stock configuration or maybe change out the TIM in the future.

Predicted Performance Improvement vs Cost

So what happens next? What will be the improvement I hope to achieve, with Zach's help btw? Hotter doesn't equal better. Cooling the CPU will affect 3 performance factors:

• We'll get a cooler operating computer which will increase its lifespan, ~$$$$.
• We'll gain back the Turbo Boost frequency loss under sustained load, ~+12.5%
• We'll gain back the performance from throttled Turbo Boost hitting 113 Watts. This is for an unknown time frame, but typically a max of 28 seconds. If that's the case, it means ~38% performance boost, equivalent to 11 physical cores, during that time frame

The cost delta of the i3 vs i5 is $100. The cost delta of the i3 v i7 is $300. These are equivalent to ~1/8th & ~3/8th the cost of the Mac mini. Coincidently, ~12.5% and ~37.5% of the base cost of the computer for the successive upgrades. If you go with the former, the cooling upgrade only nets its namesake, cooling, as there is no performance differential with the i5 outside of going to the extreme and using a galinstan-based TIM. This leaves, in actuality, the delta of the i5 vs i7, $200 and 25% of the cost of the computer, and thus my general cost marker for this project.

I can say now, that doing a one-off of a modified heat pipe variation of the cooling solution will cost about $175, but I'm not sure it'll be able to achieve the performance goal of reducing thermal resistance in half. It'll be aided some nickel plating to use 100In solder (~80 W/(m-K)) instead of 58Bi42Sn solder (~20 W/(m-K)) and then the whole assembly copper plated as Zach has done already. This replaces what I believe to be an e-coating on the cooling solution. I may test and add a thinned spray paint finish to see if the increased emissivity can in fact help the fin stack radiator, or if it is hurting performance.

Fall Back Plan - $400-$600

If that fails to work, then I'll quite possibly resort to the last ditch effort of exotic cooling if another idea doesn't present. It'll cost ~$170 to make ~95 ml of a custom galinstan alloy and I need a little more than that. At extremely low flow rates, ~13 ml/sec, and a simple copper pipe loop, way simpler than micro-channel impingement of current water blocks, 70-ish watts of heat created ~45 °C gain over ambient. Doubling flow rate reduced that by 6 °C. Using better heat transfer in the cold plate and forced convection on the radiator will dramatically improve the performance. Add in a all copper radiator for the fluid, a pump, a new radiator for the internal fan... The downside to this complexity is added cost, probability of failure, and lack of [Apple] aesthetics.