I think it would be wise for you to follow the vet's r/x. My cat's had many urinary tract infections all because I wouldn't stick with the prescribed food. Because it was pricier, I thought I could go the cheaper route and be ok. But in the end, I spent more money than I would have, caused my cat more suffering than I shoud've and myself stress that could've been avoided. It's been many years now that I have kept him on the prescribed food and he's never had a ut since.
They prescribed your pet this for a reason and I recommend, for you and your pet's ultimate good, that you follow it.