I'll go the other way. If you already have the TIW card, you are probably best off skipping any dining plan.
The QSDP is a very poor value when used for breakfast. So you'd have trouble getting fair value out of using 2 QS's if 1 of them is for breakfast, per day.
So looking at Mickey's Wife's math --- Assuming you simply used your TIW card for the same meals as she calculated -- Your TS cost would be $549.54
The DDP covers 1 QS per night --- So now it really depends how you want to use those credits. If you use them for breakfast, you'll only get about $10 per value per adult, and about $6 value per child, per meal. That's only about $200 in QS value, even with tax.
So using the TIW and 1 breakfast QS OOP -- Total cost is -- $750
Versus $834 for the DDP. (According to Mikey's Wife, I didn't double check).
Assuming use used the QS credits for lunch, and got beverages and desserts with every meal -- then you are looking at about $16 in value per adult, and $5.50 per child, plus tax -- So looking at about $275 in value.
So if you maximize your QS a bit more -- Then $549 + $275 == About $825.
So closer to breaking even with the dining plan.
And of course, the nice thing about the TIW card, is it gives you a discount on everything at the covered restaurants -- alcohol, appetizers, etc. You get a cheap meal on the DDP, and your savings disappear. Get a cheap meal on the TIW, you still save 20%. Get an appetizer on the DDP, and you pay full price OOP. On the TIW, get a discount.