We're running 1/4" worth of spacers all the way around.
Theoretically, widening of the track increases the tire's leverage on the spring, thereby reducing the actual spring rate at the wheel.
In reality, the change is small enough that you will not be aware of the change in spring rate.
However, we definitley could tell the difference when we ran different offsets front and rear, so you will probably want to use the same number of spacers on every corner. That said, we have not tried every combination of tire and wheel out there, so your mileage may vary.
I'm sure the extra tire has a more profound effect on the car's handling than the extra offset.
As far as wheels go, I believe your only real option at the moment is the SSR.
I am currently working with another wheel manufacturer to bring you something light and strong, but I can't say any more right now.