Which 125's are you refering to? Double overhead cam's is refering to the fact that there are two camshafts operating the valves. Usually, when you have double overhead cams, its because they are using 4, or even 5 valves, per cylinder. Like the YZF... its dual overhead cam, one cam operates the 3 intake valves, the other operates the 2 exaught valves. (YES, I CAN'T SPELL EXAUGHT) The thought behind it is that the multiple, smaller, valves allow fuel to enter and exit the engine better, hence more preformance. The lighter valves can also therfore sustain rpm better, because they develop less momentum, because they weight less. Single overhead cam usually incorperates a single valve for intake and a single valve for exaught. So double overhead cam is definatly higher preformance. Its also much more complicated to work on. (ie two camshafts to line up when rebuilding, etc. etc.)