As far as I know, lightning edition is already overclock up to a higher point than boost edition. It is just terminology of MSI to define their gpu's.
Lets have a look at MSI original explanation for the terminology....http://event.msi.com/vga/2012/series-comparison/
Lightning is the fastest of all with more feature and custom cooler
TwinFrozr is the mildly overclocked reference version with custom cooler
Lightning edition should be higher clocked than boost edition of reference or twinfrozr
7970 Lightning GHZhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814127736&Tpk=7970%20lightning&IsVirtualParent=1
7970 TwinFrozr Boostedhttp://www.newegg.com/Product/Product.aspx?Item=N82E16814127732