No, I am correct. DMG per shot may be higher with very high PD, but since you will not be hitting anything, and shooting much slower, DAMAGE BEING DONE IS LESS. My OP does NOT state DAMAGE PER SHOT, simply dmg. When you Assume, you make an ass of u and me.
First off your original example is shitty. a 15/27 build is a pure archer build, a 21/15 build has points left over for max PS & 5 IF. If you are going to compare a pure archer with another strength oriented pure archer, comparing 15/27 to 21/21 or 21/18 is much better.
Second you are forgetting an important fact: the less damage you do, the more shots need to hit in order to kill. So yes you might miss more often with less wpf, but a high wpf build would need to compensate for it's low damage output by a MUCH higher amount of accuracy. 17-26 wpf isn't nearly enough accuracy to compensate for the lowered damage.
Third, here are some numbers
Using
http://alpha-lider19.ru/MB/?go=archercalc :
21/21 naked archer w/ hornbow & bodkins vs someone with 50 body armor: 16.4 damage on average (this one has 165 wpf)
21/18 naked archer w/ hornbow & bodkins vs someone with 50 body armor: 15.7 damage on average (this one has 156 wpf)
15/27 naked archer w/ hornbow & bodkins vs someone with 50 body armor: 11.8 damage on average (this one has 182 wpf)
It seems, good sir, that it may be YOU who are making the assumptions here.