So, I did some testing - using skeleton spawners in the nether (for withers) vs soulshard wither skeleton T5, in 1.5.1:
Regular Skeleton Spawner, killed by regular iron golems: Coal, bones
Regular Skeleton Spawner, killed by MFR Grinder: Coal, bones, wither skeleton head
Regular Skeleton Spawner, killed by me: Coal, bones, stone sword, wither skeleton head, minium shard
Soulshard Wither Skeleton T5, killed by regular iron golems: Coal, bones
Soulshard Wither Skeleton T5, killed by MFR Grinder: Coal, bones, wither skeleton head
Soulshard Wither Skeleton T5, killed by me: Coal, bones, wither skeleton head, stone sword, minium shard
For both the grinders and kills by me from the regular spawner, I also goth various bows, enchanted bows, arrows and random bits of enchanted armour from the normal skeletons mixed in. I tried putting visors on the iron golems, but it crashed my server, so I wasn't able to test those.
Using version 2.5.3B1 of MFR, 1.27 of SoulShards and 3.0.4c of Thaumcraft.