Sirex,
Well there are actually many factors to take into account. Here are just a few things to think about.

Square inches - You're wings are only so big, you can't increase the amount of wing you have. So when you increase weight of the plane you increase the wing loading. When you increase the wing loading you change several critical flying characteristics like higher landing speeds, longer take off runs, and overall more sluggish plane. You should always strive to have the lowest wing loading you can possibly get.

Structural stress - As you increase the weight of the plane you increase the stress on all of the major structural components of the plane. Higher weight means that even simple aerobatics can stress wing components to the breaking point. Higher weight means that every landing puts more stress on the fuselage, and a rough landing has more chance of breaking something. Unless you have taken measures to strengthen the structural components of the plane you should really reconsider adding as much weight as you have.

Centrifugal force - When you add dead weight to one end of the plane you will change the way the plane behaves in circular movements. This can have dramatic effects on a plane, and usually not in good ways. Here's a simple experiment to demonstrate what I mean. Take a yard stick and put it in your hand. Now swing your arm down in a circular motion. As the stick travels downward stop the motion as quickly as you can. Not so hard right?? Now tape a pound of lead to the end of the stick and repeat the same motion. Now it's just a bit harder to get it stopped!!!!! The same thing happens with your plane in the air with dead weight in it.

Radio components - Now if you take all of the above effects and add them up they all put added stress on your radio. So now you have to either put in higher capacity servos, or face the very real possibility of blowing servos out while in flight.

I know that others will add to this list, but this what I can think of right off the top of my head. A pilot should always strive to make their plane as light as possible. Remember, gravity is THE BIGGEST ENEMY of airplanes. The more weight you have, the more that gravity is going to affect you. A case in point. Last year I built a GP Slow Poke 40. I had read that most pilots had to add 8-12 oz of weight to the nose of the plane to get it to balance the plane. During construction I did everything I could to lighten the tail of the plane. Between that and careful placement of the battery, receiver, and servos I only had to add 2 ounces to get the plane to balance. I was rewarded with a great flying plane that is a true joy to fly. That's really what all this boils down to. Whether you add a bunch of weight or not will determine if you have an overweight plane that is a pig in the air, or if you have a lightweight plane that will fly circles around others!!!

Hope this helps

Ken