1) Older laptops can be very fussy with memory. Had two laptops that I had compatability issues with when selecting memory wityh the SAME freq and timing. For my older core2due, I even first emaild the Memory Manuf - how state, Not problem should work great - You quested it, No workee.
2) Most laptops use a "limited/locked" bios and do not allow changing the timings/freq/voltage. This means that most modules that are at a higher freq will still only run at the default speeds/timings.
3) Many older laptops have one memory module soldered in (manuf defualt selling model), and one slot that could be used. In this case it is Highly recommended that the user replacable modul have the same timings/freq/voltage as the soldered in module.