Heat sink, how much does fin and base width matter?
Rather than ask about how such a heat sink would work, why don't you try actually computing it? http://www.heatsinkcalculator.com/calculator.html will let you design your own.

Note that I entered "heat sink design software" into Google, and that was the third or fourth item on the list.

Let's take your example. You did not specify the number of fins, so I picked 10. The calculator says your 40 x 40 mm, 50 watt Peltiers will operate at about 160 C. Is that a good heat sink or not? Since do not provide a quantified performance goal, there is no way tell for sure, but most likely Not.