I believe the increase in income was 10% per quill of acumen, so if your king has only 3 quills instead of 6 the difference in income should be 30% or even less. I am not quite sure here.
Piety just as command and acumen I think determines the initial influence of your king, when he ascends to the throne. I am not sure what effect it could have on loyalty - I have never bothered to check. It is important, however, in determining how many units go into a crusade or jihad - the higher the piety, the more units you get. Unfortunately I am not sure if this also determines the quality of the units, but it most deffinetly determines the quantity. From time to time, the Pope tends to give out money to particularly pious rulers. The Hungarians in the early game are a good example, and they usually receive this papal support a few times at elast, especially if they expand into neghboring lands with predominantly non-catholic population - such as Serbia, Wallachia and Moldova. The starting English king in early I think also has a lot of crosses and I have had the Pope make a nice little donation to my cause when playing with the English.

I am wondering - how do you increase a ruler's piety apart from launching sucvcessful crusades and jihads?