I feel that trying to passively cool a 100W+ TDP CPU is foolish. Even with the use of lots of heatpipes to transfer the heat to exterior fins, ambient room temperatures and the amount of air flow in the room matters. Of course, pretty much any overclocking will be out of the question so it only make sense to go with lower TDP CPUs.
The next thing to consider is that even if the heatpipes are really efficient at transferring the heat to external fins on the case, it will just mean a really hot case. Depending on how hot the CPU is, this can mean a case that can heat up your room. At the end of the day, the heat has to go somewhere...
In regards to having the PSU on top of the CPU, I think it really depends on how efficient the heatpipes are at transferring the heat and how much separation there is between the the heatpipes and the bottom of the PSU. Those new SFX PSUs are really tiny and high quality, modern PSUs can run really cool. The fan of my PSU doesn't spin up until power draw exceeds 425W and it remains quite cool to the touch at those levels even with the fan off.
Note:
When I'm stress testing an overclock, the ambient room temperatures will have a noticeable impact on temperatures. That being said, even under Prime95 testing, my Noctua cooler is always cool to the touch. That's just an example of how good airflow and effective heat transfer can dissipate heat.
Note 2:
I have an i5-6400T that I've tried to run with a stock Intel cooler (these things are really terrible, btw) and unplugged the fan to see what would happen. I don't remember exact temperatures, but it never reached extreme temps while I was doing a bit of browsing. I did not try anything too intensive and only tested a few minutes.
I had intended to delid the CPU and try with a better cooler but I got sidetracked and never got to doing it... I may revisit this little experiment soon...