If I am not mistaken the DVD write speed and time goes as such:
All are with a full 4.4 Gigs of data on a DVD 4.7:
16x DVD+/-R = ~6 minutes
8x DVD+/-R = ~12 minutes
4x DVD+/-R = ~24 minutes
2x DVD +/-R = ~48 minutes
1x DVD +/-R = ~96 minutes
If you have less data it is obvious the time to burn will be less.
By the way you do realize that your PMG5 SP has an 8x and the iMac G5 has a 4x right. From what you have stated it seems that your 12 minute burn will coincide with the 8x as you do have and your brothers iMac G5 comes in at double that speed which coincides with the 4x. So there is nothing wrong with the SuperDrive. Could be the media that is multi speed, since some manufactures label the speed of the DVD that will burn at the most accurate speed rate.