Most of the "coolth" is in the latent heat of the phase change to solid, rather than the temperature. Thus if you put a 0F chunk of food in the frig to thaw, or a 25F one, I don't expect the time to fully thaw it would be all that different.
I also suspect that the reason your freezer didn't go above 30F during the power-off periods is because a slight thawing of some foods would suck the incoming warmth and prevent further temperature rise. Items that are not distilled water, but rather have some dissolved salts in them, i.e., most foods, freeze (and thaw) at a temperature somewhat lower than 32F (0C).
A way to test my theory is to lower your thermostat to, e.g., 20F instead of 25F, and see if an 8-hour power-off would still cause a 5F rise to 25F, or would it still go up to about 30F. In the case of my freezer the temp rise was 20F and I assume both of these are standard freezers with similar insulation?
The temp gradient matters too, you had an initial diff of 55F between the inside and outside of the freezer, and a final diff of 50F, I had only slightly more than that to start with, and less than that at the end, so that does not explain my 4x larger temp rise.