Like Gews said, just add the lapua (with a few tweaks). Gews Topic: http://dayzmod.com/forum/index.php?/topic/136058-50s-gone-add-the-lapua-with-2-changes/#entry1328033
Those are pretty decent specs considering that your only getting 18-27fps. Make sure to check for any programs that have malware, or even delete unwanted programs. This is the best advice I can give, sorry if it doesn't help bro:/