I heat it until the color changes, but I DO NOT char it. You have to heat it slowly and deeply to make the change.
I could easily see it adding 5-7 lbs on a bow, but probably not a light one, 7 lbs on a 35# bow would be a high percentage of the weight, but adding 7 lbs to a 60 lb bow, nice and wide, might happen.
If done properly toasting WILL add some weight and will prevent or reverse a certain amount of set. I have taken bows with 2" of set, toasted them over a form into 2" reflex, and shot the bow in, keeping 3/4" to 1" reflex. You have to do it right, but it definitely improves performance.