There's no reason why you can't quilt that size on your Sweet 16.
However, the question to me is how are you going to baste it together, spray basting? pins? LA ? or?
For me, I need the backing to be more than the same size as the front when spray basting.
For me, that would be just too precise to be able to get it smack-dab and on the spot!!!
To say the least about the awkwardness of working with a quilt top that is that big.
For me, I would probably end up with the quilt top off the edge of the backing and batting .....
Likewise the same issues if pin basting.
As the others have suggested, if you were loading it into a LA to baste it together before quilting on your Sweet 16, you would need the extra fabric for loading it.
As you suggested, you can always add on some fake strips around the edge to give the size and remove later. I've done that and it works just fine, but there is still the challenge of getting the quilt centred onto the backing.
RhondaK's idea of sacrificing the border size could give you some extra wiggle room.
That is, if you are OK with the smaller outer border.
Though if it is for a king bed, you maybe need all that size, and then some for a good covering.
Plus, keep in mind that a quilt shrinks in size, just from the quilting. Again with washing.
For me, and for simplicity's sake, I would probably see if I could get some more of the same material as the backing, and make it larger over all. Better to be safe than sorry! ...... Or abandon that backing and get something else that I could make the size needed.
Good Luck!