It depends a lot on the system. If it is a fairly tight separate sanitary sewer system, I would guess you wouldn't see a big jump in flow. A combined, or partially combined system with a fair amount of infiltration can increase to 10 times the amount of dry weather flow or more. This creates a "first flush" load to the treatment facility and a fairly diluted load thereafter. Of course, heavy rain can be a very broad term. You can have fast heavy downpours of 1-2 inches, or longer duration storms totaling 4- 5 inches. Each one can affect the sewer system differently. I hope this helps.
Depends on how much I&I that is let in the system, form Manhole, Cleanout or broken lines and other situations. What ever flow that your Pumping Station is set up for in additional flow will give you a high wet well alarm due to to much flow.