172W vs 156W
 

Is there really a noticable difference??

How much gain does running 2x172W Pelts actually buy you?

depends on the cpu, assuming an athlon there's a fairly small diff between 156 and 172, assuming 100W of heat you get about a 25C delta for 156W and a 29C delta for 172W, so a few degrees.
As for 2 172Ws compared to one (I assume that's what you are asking), that's more like 50C delta so you do get a significant improvement.
If you're running a P3 or other low wattage cpu the improvements from large peltiers is less of course.

What happens when you try and run a 24V pelt at some lower voltage?

You get proportionally less cooling with lower voltages, so running a 24V pelt at 12V gives half the cooling power - a 172W becomes an 86W etc.

A 172W Peltier would give some temperature difference over a 156W one, as the 172W Peltier is measured 40mmx40mm, whereas the 156W Peltier is 50mmx50mm. The 172W Peltier will have a smaller surface area, meaning that the 172W will be more focused on a smaller area, which will give lower temperatures.

Sometimes operating a Peltier at full Voltage may not be a good idea. By running a Peltier at its QMax and VMax will add lots of heat, I mean LOTS of heat to the hot side, making it very difficult to cool, especially in the case of the 172W Peltier. By running the 172W Peltier at something like 150W, you get a cooler hot side temperature, thus, possibly a lower cool side temperature. Yes, it all seems complex, but you'll get it eventually.

However, if you think you have an EXTREMELY good cooling solution, such as a Watercooling/Fluorinet setup inside a Freezer, you should be able to run the Peltier at VMax without any probs.