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.
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