ATC watcher / Clandestine
The amount of weight a repair adds to an area of structure largely depends on the skill of the guy doing the job, there will always be some extra weight but it will always be a lot less in percentage terms than a metal repair.
Scarf joints are usually stronger than the original material by a small percentage, on a wooden aircraft you always make a test sample using off cuts of the wood you have used and the glue, once the glue has cured you take a hammer to the joint, if the wood fails that is good, if the glue fails then you have to re-do the job.
Composite work is much the same as wood when it comes to scarf joints except the glue is replaced by resin that is exactly the same as the original structure so as long as you have room to get the correct amount of scarf area there is no need for for extra structure as all you are doing is replacing like with like.
The most exacting repairs are on glider flying controls were the allowable extra weight is measured in grams, depending on how well the original manufacture was will determine if the control surface has enough margin to make a repair.