If no machine work is being done to the pistons or rods or crank or flywheel, then there is no need to get it re-balanced. What is the rev limit of the 20valve? What is the rev limit on your Mums car? Compare the two. The 20valve is probably the better built motor right?
You are going to the trouble of porting, with stock ECU on the street? Not sure what the gains would be. Better money spent on new rings and bearings.