If you add a topsail, do you not need to increase the height of the mast? If you are going to make the mast longer, why not just have a sprit sail with a longer luff? That way the center of effort will not move much if any.
There is a practical limit to how fast you can make an Opti go. If you add power beyond that which the little boat can use..... you have not gained much, if anything, aside from the satisfaction of having exceeded the bounds of practicality.
As you say, you have outgrown the boat. If that is to be taken to mean an increase in the skippers weight, then there are some arguments that you can make about power to weight ratio which also has its limitations in a boat like the Opti.