Lines that are used for Spey casting are quite different in design than normal single hand WF lines that are used for overhead casting.
For Spey casting, more weight/mass is needed in the rear portion of the lines belly to load the rod when the "D Loop" is formed. Most Spey lines also need a longer front taper so that the front portion of the line that is lying on the water, creating an "Anchor", will lift off the surface easily or the forward cast will loose its energy from the stick of the line too the waters surface tension. The longer the line, the longer the front taper will usually need to be, or the cast will lose its forward energy.
Skagit lines that have very little or no front taper, are made shorter and really heavy in the rear portion of the belly, to load the rod deeply and minimize the amount of line Anchoring on the water at the start of the forward cast, which allows for a powerfull turn over. The new Compact/Short Skagit heads, in both single hand and two hand sizes, have achieved a nice balance of mass/taper/length. For single hand Spey casting, it's equally as important to keep the rod/head length ratio in balance, as it is when two hand spey casting. Most regular WF lines have head lengths that are much too long for effective spey casts with single hand rods. Also, the weight/mass of single hand WF lines is too evenly distributed through the belly and front tapers, making the rear portion of the bellys too light to create a D Loop that will have sufficient mass to load the rod effectively.
I really like the specs of the new SA Skagit Single Multi Tip line for Spey casting with single handers and switch rods.
http://solutions.3m.com/wps/portal/3M/en_U...leHandMultiTip/