Getting rid of the filler plates is definately performance enhancing for soybeans. 

I've never seen one of our 9600's be speed limited by anything other then engine hp or cutterbar performance in beans with the exception of some 'early beans early' which stopped growing in July...right before a 'too late' rain brought the waterhemp it's second wind, with nothing to compete with anymore...I discovered straw chopper capacity was limiting there, after ignoring an occasional chirp from the chopper shaft monitor all day assuming it was a bad connection.  Turned out it wasn't a bad connection.   The walkers are not very comfortable in August in Kansas.  If the shoe is limiting I would consider that an adjustment issue.   Shoe was almost always limiting on a 7720...I don't miss that.