Theoretically the first 30 feet have the same mass. If the front tapers are the same, they will cast identically for the fits 30 feet. HOWEVER, there are WF lines that do NOT have the same taper as the DT line. Then they will not cast the same.

After 30 feet, it also depends whether the WF line is a long belly line that has the rear taper and running line place further back in the fly line. So you need to look at the actual fly line taper to tell when the rear taper and running line of the WF line starts. That is when the mass of the DT and WF begin to diverge.

Note in the taper chart below, the Orivis Wonderline Standard WF and Easy Mending (Long Belly) have dfferent lengths for the belly and the rear tapers. Therfore, they are both WF lines but they will not cast the same at all casting lengths.