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.
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