I would go to a 3 disc set if you are duplicating these. (3) DVD-Rs are cheaper than (1) DVD+R DL, The case may balance out the cost but it will seem like more of a value, quality will be better. Remember even 2-2.5 hours on a disc is pushing quality, footage from GL2 tends to be grainy.
Remember a DVD DL is not equal to (2) DVD-Rs, it is less. 8.5GB vs 9.4GB. Edit anything you can out of these, most things can be tightened up quite a bit.
Here is the break down on Bitrates if you use ac3 audio.
3.8 Mbps – 270 minutes on a DVD-9
4.3 Mbps – 240 minutes on a DVD-9
4.8 Mbps – 120 minutes on a DVD-5
3.8 Mbps – 150 minutes on a DVD-5
tough to get a very clean encode with compressor at 3.8 Mbps unless you are work with perfectly exposed Dbeta, and even at that it is pushing it.