With only one panel, our solar fan didn't help a lot. When I added the second panel, it seems that it performed much better...didn't actually measure the temp though. This is in the lower attic that is maybe 600sq ft of ceiling max (only the master bed and master bath). I'm satisfied with it.
When I get around to it, I'm adding more cellulose in the upper attic, and maybe a solar fan or two as well. While it is always warmer upstairs, it seems DD's room is not just warm, it is hot, so I'm thinking there is a thin patch of insulation over her room.
We have a whole house fan as well, but can't use it in the middle of the day or we just bring in the hot air. Rather, we turn it on at night and early morning, cool the house down to about 68F, and it stays under 78 until 4-5pm on most days unless we have 105F++ outside.
R