Using Shift - cast spell doesnt queue for spellcasting, it will instead just go cast the spell and ignore what it was doing earlier.
Similar after giving a spellcasting command, Shift - cast spell again doesnt queue for another spell.
While this being true there is a very good way to declone spellcasters for mass spellcast, say Yamato or Irradiate, in a very short distance very quickly.
Select your 12 BC's (as an example), hold Shift at all times (for me this is done with pinky on left-Shift), click Y for Yamato and select your target (Mass Turrets or whatever), as you are already holding Shift you are already in decloning mode (minus one BC) by clicking any already selected BC in the selection window, re-cast ('Y')amato on another target while keeping Shift held in at all times until you are done casting all 12 Yamato's.
To make this even faster you can keep your intended targets as close to the bottom screen as possible to make it a short path from target to declone.
When you are done decloning and want a safe way home you can save your BC-group in any hotkey before casting the spells, click that hotkey group and Shift - rightclick anywhere to make them go there after casting the spell. (Poor example as you might be wanting to fight now ^-^)
Here is a map if you want to practice it. If you do it in SinglePlayer you can F10-E-R to restart quickly. The distance in the map is fairly short to simulate a very close decloning, you rarely have that much space to do it in a regular game anyway.
EDIT: To make it more clear. The keynote here is Holding Shift At All Times, which might seem counter-intuitive when you think about Shift queues commands, it does not for spells so thats why you dont have to ever lift your finger from Shift while doing this, which i guess many of you have done all along because its key feature "queue" which did in my mind feel right to lift off but now i gotta re-wire my brain.
EDIT2: The word DeClone is probably wrong here as thats more of a hotkey pairing thing (hotkey cloning). But at least you understand what im getting at.