To be honest it's a lot more economical to have cheap large mirrors on your roof directing sunlight towards a concentrated spot of PV cells of higher efficiency that are somewhat expensive.

You could use some of that energy to have tiny motors that track the sunlight.

This would net in more energy and less cost per square meter right?

I've done a lot of experimenting with that. Here are some of the (surprising!) results:

  - you only need one motor to aim a whole array of mirrors
    if you're willing to make some exotic gears

  - the lifespan of the cells will shorten measurably

  - you will need to cool the cells if you don't want them
    to die very quickly

  - there will be a serious fire risk

Are there cells designed for higher intensity than others -- ones that can handle the extra light and maintain efficiency?

Yes, but they're not cheap, in fact they are so expensive that it will be cheaper to cover the mirrors with cheaper cells directly...

At this point in time there are no cheap and easy solutions in sight to make the next 2 to 5% gain in efficiency in a cost effective manner.

But given enough time and effort I'm sure that that will happen, I expect the final cut-off to be somewhere around 25% net efficiency (solar incidence to electricity out of the system) for smaller installations and maybe up to 30% for much larger installations.

Yes, there are different cell and cooling designs for low concentrating PV and high concentrating PV (LCPV, HCPV). But the price of non-concentrating PV systems has, so far, fallen fast enough to prevent either LCPV or HCPV from taking even 1% of the PV market. Soitec, Amonix, Suncore, and Sunfocus all failed to drum up enough CPV business to stay solvent.

Maintenance and installation costs already eclipse the panel costs. For any assembly you evaluate, you have to answer two questions:

Will this require more complex installation than flat panels?

Will this require more lifetime maintenance than flat panels?

If the answer to either question is a yes, it doesn't matter how good your system is in any other measure, you will lose against simple flat panels in total cost per electricity produced for any small-scale installation.

Correct. I found this out the hard way with my two leafs of 2x4 panels tracking the sun in two axis. By the time the system was up and running I could have had more power and less headaches by installing 24 panels statically on the roof. It would have looked better too.

Tracking systems with moving parts are expensive and complicated. With good quality 320W panels at $0.45/watt now in pallet quantities, going with a low cost fixed mounting system (whether ground or roof) makes much more economic sense. Considering the wind loading that a two axis tracking system which can hold 6x280W panels must endure, no wonder they cost a lot.

That sounds like the "more economical" is wiped out as soon as you have a maintenance issue in the moving machinery. As it stands, maintenance-free panels are a huge advantage.