Swiftshader from Transgaming may increase youre FPS. However, the lagg generated in WoW is not only caused by Direct 3D.
As an example, WoW decompresses textures, sounds, models and maps from the MPQ datafiles. Remember the loud noise youre harddisk is making upon entering a crowded area? WoW is reading all kind of data from the compressed archives at this point.
One thing that realy improves my FPS while doing end-game raids is turning all sound off. Not just turning the volume down, no, turning everything off (uncheck all boxes in the sound options.)
Remember all those people disconnecting at the 'Razorgore the Untaimed' encounter? Muting all sounds realy made a big difference for the people in my guild.
So, unless you have an old graphical card (or onboard card?), Swiftshader wont increase youre FPS that much in the World of Warcraft game.
Just my two cents.
Siker.