Don't listen to ntc3freak...
For a good, reliable engine, you are best to look at O.S.
I am going to presume that since you are still running the GX15 that you are a novice, nothing wrong with that, I am too! I guarantee that if you and a friend had equal skills, your friend had an O.S. TZ, and you had an O.S. TR, you would probably beat him, even though you have slightly less power, power doesn't win many races, control and consistency does. More power will just force you to make mistakes.
Having said that, for club type racing I'd say the O.S. .12 CV-R would be best for competitive, yet easy to drive performance.
Talk to some people at your local track, ask them what engines they are running.
Oh and I wouldn't buy any engine from an Australian Hobby Shop unless you love to burn money, they rip you off a disgusting amount.