I am a native English speaker and was in a similar boat to you, with my 'superior' English I would get 60 points, which was a scrape for my visa. I did the testing thinking that I was English, what could go wrong?!?! I got 7.5,8,8.5,9, ie. not superior. The biggest thing for me was not knowing what to expect. It was extremely stupid of me to expect that I would just get it. I went home, re-booked (another £130) and this time, did many examples of listening (my biggest weakness), reading and writing. On repeat I got 8.5,8.5,8.5,9, superior So my advice is, practice, practice. Results don't take long, 2 weeks at the most, not enough to delay anything imho.
WRT agent, I used an agent as they needed to help me with an RPL and once I had done that, it was *only* another A$1600 for the rest of the visa which I thought was OK as I didn't want mistakes. My agent was awesome and did all the heavy lifting. The biggest thing an agent can offer is translating laws. Remember that your understanding of a requirement is not necessarily what Aussie immigration understands it as. My advice on agents is this, if you can afford one, do it. If you have a good agent, they take away a great deal of stress. If you don't have the money, you need to do a lot of reading to make sure you really understand what you are doing. Not tricky, just takes time, something you don't sound like you have a lot of.