This is a reasonable assumption.
Do we have data from other people who have been speeding through this intersection (if the sign was occluded that day, certainly it was many other days prior, too) and hit someone?
A search of that local news site for NE First Street crash reveals nothing. And that's the thing. Most folks don't go more than 8-10 over, for a speed of 33-35 in a 25, especially in a residential area.
So, look at it from a Feet Per Second situation.
25 = 36 FPS
35 = 51 FPS
51 (spotting him a couple mph) = 74fps, or more than double the speed limit.
So, a 25 mph speed would mean that if you saw the stop sigh at 35 feet away, you might have hit the brakes just as you are about to hit, given average human reaction times, but not enough to not change the fact that you were going to crash. One thing is that he's carrying a lot more energy into the crash than a 25 or 35. Like this
5,000lbs moving 25mph = 141,638 Joules
5,000lbs moving 35mph = 277,610 Joules
5,000lbs moving 50mph = 566,550 Joules
So, dispassionately speaking, the impact was more than three times as bad as it should have been. Or twice if he was going only 10 over like most folks.
But not matter how bad the tree was, there are those 35-45 feet between the tree and the intersection. But here's the thing..... see that big fat line? That's called a stop line, where you are supposed to stop for the stop sign you cant see. An expert, highly aware driver should see that. This guy was cut every possible slack, and then some because of what he wears to work.