Is the stock tint a film or is in actually in the glass? I tried to look and could not tell, but would lean towards the latter. I would probably not mess with putting any over the stock tint for a couple of reasons. It looks like it is 5% tint and if you get any darker, you might have problems seeing at night. Also, if you put tint over it, and it is film, you would have to remove the two layers when replacing it or if any problems arise (bubbles,fading, etc). Has anyone found a tint that matches the factory tint? I have only ever had LLUMAR platinum plus and they don't have a color that matches it. Lastly, make sure you are tinting with in the legal parameters of your state.
One last question, how much are you guys being quoted for the tint on just the front two windows?