Trick question. Neither are correct. Should be Stephens' daughters.
Assuming the lack of capitalization was an accident, it depends on what you're referring to. If it's a family whose last name is Stephens, then it's the Stephens's daughters. If it's one person's first name, then it's Stephen's daughters.
issues might occur like, 1) Massive performance drop 2)Lower frequency ram will try to keep up with higher one leading to more heat and stress on ram more than its designed ability. 3)chances of shorting or damaging the motherboard.