As you can see by the save log on the right, it took a bit of effort to get the simulation of the machined coaxial hotend to generate believable results. I was having ABS filament that seemed to be uncooled, even though the contacts seemed to be set up correctly:

As you can see, the cooling performance difference is drastic. 

For the machined coaxial hotend, assuming that the 6028 blows over a CSA of 36mm x 15mm, I got a coefficient of convection to be 32.3W/(m^2 K) from a wind speed of 10m/s, implying that I could run the fan at half-speed and still have adequate cooling. I used 0.03 to be the emmisivity of radiation. However, the conduction through the heatbreaks in the unibody is much too high and thus the entire part is still over 100C.

Going from the simulation, it seems that for a nozzle temperature of 250C, the lowest I'd be able to run the coaxial block is 160C (which is also the temp at which PLA starts to melt). Additionally, not much heat is lost from the nozzle block, unlike the unibody hotend.