The 20 mpp was an example number, but in the range of reality.
Each ship able to raid some mpp, based on strength, xp, tech, supply and readiness. Subs have a much higher value, than other ships, and big ships have higher than small ones. A good sub can do 20 mpp range raiding, other ships will do only in the range of 10.
The US war entry can go up, is German units are on the UK spots, It will go up faster, if unconditional sub warfare is allowed in the region, and you can trigger the Lusitania event which will give a one time boost for US entry. It can go down is the Entente do the German blockade, but with much smaller chance, so if both done it will raise in general, but slower than if only Germany doing bad thing to the US merchants.