AFAIK Letho's @ 1GHz is OPP5 = same as 600MHz on stock

my ULV kernel voltages compared to stock:
my <=250 is <0 stock (ultra low voltage)
my 500 is ca. 0 stock (lower voltage)
my 550/600 is 250 stock (low voltage)
my 700/750/800 is <500 stock (low voltage)
my 850 is 500 stock (normal voltage)
my 900/950 is 550 stock (overvoltage)
my >=1000 is 600 stock (overvoltage)

does the ULV kernel work stable with 850/900/950/1000 for you?