Carter,
That seems like it would take longer... I believe a storage VMotion from thick to thin should take less time than thick to thick (i think).
Also, would the -StorageFormat be available hot? As it stands, you cannot "inflate" a hard disk from the GUI hot - you have to Storage VMotion to do that.
If the Move-VM command took the same amount of time, and you could do Set-HardDisk hot... that would be fine. Otherwise being able to set the option in Move-VM like..
Move-VM -Datastore Y -StorageFormat Thin
would be best.