Generally effective edge is not doing much when you're not carving. For straight lining, a longer, stiffer board (all other factors being equal) will be better.
That said, why do you feel like you're missing straight line speed?
I've been riding more crud lately and I feel very unstable at times going fast on my medium stiff boards (TRS, Rossi Angus and Templar).
I generally prefer shorter boards and I was wondering if getting longer medium stiff board will be better or same length stiffer board will be better for these conditions at higher speed.