If the vet is sure it's a food allergy, have you tried switching her to one of the grain free wet foods? There are ones that are 95% meat from a single protien source, such as all chicken or beef or turkey.
This site was recommended on a dog forum. It gives the wonders of apple cider vinegar. There's also a section on cats. If you scroll down the page you will find some testimonials on cats and food alergy, scratching & scabs. http://www.earthclinic.com/Pets/acv_for_cats.html
Hi, up untill about 4 months ago my Tabitha was scratching her face and ears untill they bled,the vet thought it was ear mites but it wasn't. It was costing a fortune.She was so miserable and wouldn't play or anything.I was in a pet shop one day an saw Phytopet, it is a company that makes herbal remedies for animals. I bought ear drops wich contain lavender,clove and juniper and also one for her skin which is put into her food. I though i will try anything to stop her bleeding. They were 4.00 a bottle. I could not belive it within 2 weeks it all cleared up and she is now acting like a cat should do. Well a little too much now! Try Phytopet, i worked for tabbi, they have a massive range and what's great is it's tested on humans!! hope this helps