Funny you ask that while the indev Version of GT6 has the Scythe/Sense drop a guaranteed Wither Skull when killing a Wither Skeleton.
Also yes, long ago i had the Issue of not enough Wither Skeletons spawning whatsoever, i just got reminded of that by some random Lets Play recently, so I added the ability for the Scythe/Sense to always drop a Mob or Player Head upon kill, at the cost of dropping nothing else when used.