ORIGINAL: MinnFlyer
Bottom line: it is better to add weight than try to fly an unbalanced airplane.
True.. However I will interject this comment which is nothing more than my opinion,
if there is any way possible to move the cg by simply moving equipment that is already in the plane instead of adding weight, that is the route I would take provided it can be done without compromising the airframe. Examples, moving the battery pack, servos, or receiver forward or back. Once you have optimally positioned the gear and you still cannot balance THEN add weight if you need to. As has already been mentioned, add the weight as far forward or back as you POSSIBLY can, again it will require less weight if you do.
As Minn said, some weight
might help the plane fly a little more stable if you are on the low end of the recommended wing loading. But be cautious in doing so, because as Ken stated the wings can only lift so much weight for their area and airfoil design. Once the plane gets too heavy it will be sluggish and difficult to fly unless it is going faster. This means faster landings and a faster minimum flight speed, because the plane will stall at a propotionally higher speed. But up to a point (maybe a few ounces), additional weight will not be too noticeable.
Minn I am surprised that your Seawind flew well with that much weight added. That Seawind is pretty small, how did it perform after you increased the weight? What changes did you see before and after from the increased weight? I am curious!