I've heard of some folks using a Fresnel lens to concentrate light onto a smaller area, where the silicon is. It improves the efficiency significantly, but can also cause heat issues (which can hurt efficiency).

I've tried this. With standard PV panels going over 2:1 is not advisable. The panels will live a lot shorter and if you go higher than 2:1 they will break (wiring will de-solder, the plastic bits & pieces will melt and they sometimes simply stop working). I've tried watercooling as well (with very thin panels) and that works better but the lifespan reduction is still significant, you can see month-to-month performance degradation (as compared to a control that is running on 1 sun).