Concern about 'Performatrin' dog food

My dog just got over a few weeks of kidney trouble. He was drinking large volumes of water, frequently urinating in large volumes and the specific gravity of his urine was measured to be 1.009. (I understand that normally it would be about 1.035.) Various tests for bacterial agents proved to be negative; it was unclear what might be causing his diabetes insipidus.

Because he was in 'stage 2' renal failure, however, our veterinary recommended that we switch to food that is low in protein and phosphorus to reduce the load on his kidneys. She supplied us with a quantity of prescription diet based on duck.

Within about a day my dog no longer displayed any heightened thirst or need for frequent urination. Beyond that, his specific gravity is now 1.027.

I mention all this because the main change we made was from 'Performatrin natural Lamb Meal & Brown Rice Formula Adult Nutrition' to a product from Medical.

Has anyone else happened to have had similar experience with a recent batch of this product from 'Performatrin'? It is labelled 11:55 BB 24/12/11 (which we take to be a 'best before' date).

Thank you.