I'm not sure how small intestine bacterial overgrowth can be diagnosed via blood test. Are the vets sure it isn't inflammatory bowel disease?
My opinion would be to ditch all Hill's products (extremely poor quality ingredients not fit for an obligate carnivore) and find a better quality *meat-based* wet food. Raw would be ideal, but it takes much more effort and vets don't tend to be very supportive. Here is more info on how diet is so crucial to GI issues:
http://www.catnutrition.org/ibd.html