you forget a key element.
Let's say 100k pop sci gets you 5% extra pop on a dwarf.
I don't remember how high sci nw is, but let's just say those 100k sci points = 1000nw, to make it easier.
5% extra pop, which goes into army. You could get 5% extra army in another way, by growing 8% land. This is because you need peasants if you get more land.
That 8% land will cost you ALOT of nw, and surely more than 1000nw. I lost my nw calcs since my comp crashed, but I believe it was around 60-65 nw for 1acre+building.
No need to say that 5% pop sci pays off if you're 150 acres or 15000 acres. The bigger you are, the more it pays off.