Interesting concept...I've noticed I've been losing faster since early December than I was immediately before. I hadn't considered the Calories burned warming my Slushees/Slurpees up to Body temperature. I had noticed though that a Diet Pepsi Slurpee seemed to have a negative Caloric effect...which it likely does:

44 oz / 16 oz-lb = 2.75 pounds
2.75 lbs x 454 g-lb = 1248.5g = 1.2485 kg

Let's assume for this purpose a Slurpee is 0F (-18C)

1.2485 kg x (37C - -18C) = 1.2485 kg x 55C = 66.6675 kcal

Since a Diet Pepsi Slurpee only contributes 24 kcal, that means drinking one would actually burn more Calories than it contributes. Of course, the same cannot be said for Regular (Sugary) Slurpees as they contain 150g of Sugar which contributes about 600 Calories. The < 67 kcal burned heating the Slushee is only a drop in the bucket compared to the 600 kcal in the Slurpee itself.

Now, that said, a 7-11 Slurpee is likely not 0F...Though, the crushed ice Stevia Sweetened Slushees I make probably are at least that cold as they stay frozen for quite a while after being left out.