a typical usb portable hard drive consumes on the order of 2 watts. tablo ranges from 9 to 15 depending on a number of factors.
So i would estimate that moving the hard drive off of the tablo power supply will reduce the load on tablo by something like 10 but probably not more than 15 percent. a tablo that is consuming only 9 watts is not doing much, which means it's probably not hitting the disk drive (not much power draw from the USB port) . tablos that are working hard and consuming closer to the 15w figure (less for a 2-tuner) would most likely require heavy use of the disk as well, pushing it closer to 2 watts.
So, for example, of the total power consumption of a hard working 4 tuner tablo consuming 15w, 2 of those watts, or 13 percent, are powering the disk.
power corresponds directly to heat, so what we can say is that we can cause such a tablo to dissipate 13 percent less heat by offloading the hard disk. how that translates to temperature depends on the install environmental conditions. however, since most of the steady state factors involved are linear, it would not be that innacurate to say "tablo will run 13 percent cooler"