It is possible to beat a rake as high as 20% in my view.
Most of my betting is on first fours which have a rake as high as 25% (NSW TAB) and i have managed to show a healthy POT on these bet types over the past few years. I do have an advantage however of having software to generate the hundreds of combinations per race and a robust mathematical model to predict the first four placings. My biggest problem is finding large enough pools. Small pools are bad for my method of betting because dividends are "unders" when the outsiders come in. The beauty now is that exotic bets can be placed with corporate bookmakers to avoid smothering the dividend. The problem however is that they are quick to ban you when you start collecting big payouts. I have been a fulltime pro for about 4 years now and have consistently beaten a 20% rake. You just have to be a little smarter than the average person, have a decent bankroll and a good computer program. And a PhD in mathematics helps too!

Cheers
ATC