How low CL shuld you go

Hello
i got my self a new mobo and some shiny mew DDR3 ram
i youst got my Amd quad running stable at 4,0Ghz 55C
and whent ower to my ram, im pritty new to DDR and Cl clocks in general
and culd realy use some advice.

sens my cpu is realy stable and the only way for me to tinker whit my ram is frue CL or FSB i rather not tuch FSB. due to FSB change= CPU clock speed as well

standard mem
Kingston DDR3 HyperX 1600MHz
8GBKit med 4X HyperX 2GB DDR3, CL8-8-8-24, 240pin, 1.65v

My stable clocked Mem
Kingston DDR3 HyperX 1600MHz
8GBKit med 4X HyperX 2GB DDR3, CL6-8-6-24, 240pin, 1.72v

shuld i go lower CL ?
or is there a limit to the performance gain/Cl speed ?
i got a V10 cpu cooler so it got a direct 120mm fan 3cm above the ram so heat shuld not be a problem