@Nanook,
@Hifihedgehog : Both of you are missing one aspect Intel TIM. The 8700k use TIM and the 9900k is soldered that makes the difffence. If both CPUs are soldered or using TIM (like 7700k vs 8700k) you are right but not with this important difference.
A little example:
Intel i7 5820K (6 cores 12 threads) 140W TDP SOLDERED
Intel i7 7700k (4 cores 12 threads) 95W DTP TIM
It is easier to cool the 140W 5820K than the 7700k. I tested it with an Thermalright AXP100 on both CPUs. In the old Prime95 version without AVX the 5820K reached 70-75°C while the 7700K reached 85-90°C.
So a soldered CPU works on the same level as dellided CPU. The advantage is 15-20°C.
The new 9900K will be a great step for SFF because it will bring us back to the Sandy Bridge age. I think a 9900K will be coolable with an L9i and will reach 80-90°C in Prime95 and 70-80 in games & software.
TDP:
The real TDP will depend on the board you will use. Modern boards allow the CPU to run the max turbo on all cores this results in a much higher TDP between 120-150W. You can check that with running Prime95 and read the TDP in CoreTemp. Only if you setup Max Power Duration to 95W you will run the CPU on default specs. For example it doesn't matter if you buy 8700 (65W) or 8700K (95W) on default configuration the max clock difference on all cores is only 100Mhz and also the TDP is the nearly the same. Theoretical it is "easy" to cool an 8700 with the L9i if you limit the "Max Power Duration" to 65W.