Nice mate! I always wonder if these heatsinks and shrouds and all those fancy things act more like heattraps in close/confined cases like the TU100. They might be able to absord heat, but since the airflow is poor, they're unable to get rid of the heat. The only net effect is obstruction of airflow, because simply absorbing/redirecting heat doesn't make the components cooler if the heat isn't directed out of the case eventually. That's my theory, anyway
On a related note, I'm looking so much forward for my RAM/MOSFET heatsinks so I can see if I too can get some easy temperature gains by simply removing **** I don't need
That's a mighty fine looking setup you got there. I would most certainly go for an 80mm fan, if you think that is doable. I know that on the Big Shuriken 2, adding a second fan (in the case of the shuriken, it's a 60x60x10mm fan if i recall correctly) will lower temps about 2-4 degrees C. I imagine you could achieve better results on the L12 without any issues as there is better fans available for installation.
Also, don't worry about the position. Your position is pretty much optimal: when you have heatpipes, you always want to make sure that the C's are NOT(!) in the top, since this blocks the heat-directing abilities of the heatpipes - heat goes up, and if the C is in the top, there will be a gather of heat in the bend of the heatpipe . In your setup, the heatpipe C is in the bottom, so you're fine.
Also, as long as the heatsink fins are able to let go of the hot air, i.e. there is no "trap" between RAM/CPU heatsinks/Mosfet heatsinks/IO heatsinks/m.2 heatsinks, then you are fine as far as fin orientation goes.
One thing I noticed when flipping the PSU is that load peak temps went down. In my setup, the SF600 doesn't spin at all, so you might not be able to get the same results if you flip yours. AS you said yourself, it's worth trying to install the CPU fan in a pull-config, funnelling air through the PSU (like Fer has suggested).
In short: I doubt repositioning you CPU heatsink will change anything, however, toying around with the CPU fan setup and PSU setup almost certainly will show some room for improvements.Edited by ihateolives - 4/13/17 at 2:29pm