I said:
"You seem to be mixing apples with oranges- 250 w/m2 is not total ground level radiation. Might be about right if you are talking about average yearly total solar radiation you would recieve for a fixed solar panel."
You said:
"For ground level the solar irradiance varies with the season and it is even harder to find reliable information. In fact you may be correct that I am greatly underestimating ground-level sunlight."

What I saying is you are "probably" not comparing the same thing.
In other words if you were to say that on average anywhere on Earth a sq meter of ground recieves 250 w/m2 of solar energy, this would seem fairly reasonable. But it doesn't mean that the most energy you will receive per sq meter is 250 w/m2. When the sun is high in the sky, and it's clear weather, and your surface is facing the sun then you are going to get somewhere around 1300 w/m2. The atmosphere isn't a significant factor- it's the planet getting in the way (nite) and the angle in sun is shining on a the surface. Now, in LEO you still have Earth blocking the Sun and if surface isn't always pointed at the sun then you are only going to have a slight increase in total solar energy compared to the Earth surface.