I went straight for the Infernal Staff and am still unsure about the reasons to clear Whimseyshire...I'm playing my paragon 13 wizard on inferno, master level 3 and unless I'm missing something (hopefully lol) I don't see much point in doing it more.
Once you have made the staff, enter and clear the whole map in whimshire. I took the teleporter thing at the end to be sure. After that, go to the next difficulty Act 4, quest: Prime evil, Begin quest. Then go buy the next plans off Gorell the Quartermaster.
Upgrade the staff at the blacksmith, rinse and repeat.