coreboot

hey so i have a lenovo thinkpad t420 its supper slow bc its on a hdd (im going to upgrade that) anyways it used to have 4gb ram at 1333 but i added a 1600 4gb that i had in another system. however i think i want to upgrade it to 2*8 gb of ram but i want to do it as 1600 not as 1333 ive seen...