The reason for not going much higher is that it will shorten the life of the cpu. Any time you add voltage to a cpu for overclocking you are making the cpu produce more heat and while you can diaspate the heat with the heatsink it still is producing the heat. Making the cpu now run at a higher temp is ok untill you reach a certian point when it becomes damaging to the cpu and people who do benchmarking for a living have found that going past 1.38 v will shorten the life of the cpu. The cpu will still function at that voltage and at a higher vltage also , you can go up to 1.4 or 1.45 and it will still work as long as you can dissapate the heat it produces. But any voltage above 1.38 will lead to a shortened life of the cpu.
Any extra clock speed that you would get would not be worth going higher.