I tried concentrating light onto 4 small, seriesed cells from radio shack. On a sunny summer day, in direct sunlight, I got about 2.6 volts max, open voltage. Using 2 flat mirrors to concentrate the sun's rays, I got a momentary increase, then an abrupt decrease to about 2.2 to 2.3 V. After removing the mirrors, the output gradually returned to its prior level.
You should check current, too.
Current is mostly proportional to the amount of light (number of photons creating electron/hole pairs). Voltage is mostly proportional to temperature (width of bandgap minus thermal energy in electrons in the lower level).
So while raising insolation with mirrors will have little effect on the voltage (other than lowering it by heating the cells), it will boost the current, which will stay boosted.
Solar panels typically have three or so extra cells so they'll still have enough voltage to charge batteries on hot sunny days. That's why they have voltage ratings significantly above the batteries they charge (and thus both need charge controllers or automatic dump loads to prevent overcharging and deliver less than the rated power in actual battery-charging service.)
One problem with concentrators in some of the early experiments is that the extra sun heated and discolored the material bonding the cells to the glass, causing it to darken and absorb much more of the light, reducing the life of the panels and making the concentrators counter-productive. I think they abandoned them, though, because the cooling issues reduced the effectiveness of the concentrators to the point that it was cheaper per watt just to use bigger panels and simpler array designs.
(Most of the designs I've seen on the board so far, other than the oil-filled one, look like they'd have even worse cooling issues than commercial panels.)
