Try to locate a feline neurologist. This is what I did when my cat lost her balance and had difficulty walking. It turned out to be a middle ear infection and yes, she did get better.
The vet will *not* be able to see any signs of infection if it's located in the middle ear. A neurologist can diagnose this type of infection using physical tests he will do with your cat; for example, placing her on her back and watching her eye response.
My cat was put on a fairly new antibiotic, Zeniquin. During the first two weeks her balance improved, but even with the new med, she developed a slight head tilt, and though her eyes weren't dilated, there was a day when they were moving back and forth rapidly as if she were reading! I was very relieved when the rapid eye movement disappeared - it took longer for the head tilt. I was told that sometimes, unfortunately, it doesn't go away. Usually, though, unless the head is at an extreme angle, cats adapt to it pretty well.
Luckily for my kitty, the tilt did go away, but just a couple of months ago returned for almost 2 weeks. She has been on Zeniquin since first being diagnosed, not unusual for chronic ear infections. This med has made all the difference for her - never thought I'd be so happy to use an antibiotic long term!
Since your vet is already puzzled by your cat's symptoms and has prescribed treatment, it sounds like he's done all he can do. The good news is that you do see some signs of her behaving more normally. I would seek a second opinion, preferably from a neurologist, if you don't see continued improvement in another 5-7 days.