Steve is right. In that kind of setting, it's all about the obstructions, int this case, buildings. Increasing power doesn't penetrate a building.
Where I live and operate, we have lots of trees. In this case power does come into play. If you can't get above the trees(and I can't) then you have to go through them. When I'm at my house and on a UV-5R8W, if I use medium power, I can't hit my favorite repeater. On high power, I can hit the repeater just fine. When I'm near or on loop 610 with my mobile radio, it doesn't matter how much power I use, I get picket fencing. You just can't punch through buildings with a GMRS radio. You can, however, sometimes bounce off of them.
This is why trying to figure out the fars can only be done by a field test.