You are into dangerous copyright issues here - I'm not even sure if you're allowed to make a back-up copy for personal use.
There are two main problems with commercial dvds aside from the legal issue. The first as you say is that they normally have greater capacity than the blank discs. The second is that they have copy protection systems. The best place I have found for all advice on my dvd recorder is the following address.
There are forums there which deal with just about everything, and the following article deals with your query