Actually they use non-tempered for the bottom too. I called the company to confirm. I had planned on drilling the bottom, but after seeing Monkiboy's setup I think I'll go with the back.
It just so happens that I'm looking to do the same setup that you did. I got inspired by this by Petco's $1 per gallon sale. I'm picking up a 40 breeder this weekend and a 29 for the sump. I didn't realize that you could drill tempered glass. I know the Aqueon brand of tanks have a plate...