This is going by my experience.
I don't know about the other subjects, but I think for Maths that the trials were harder than the actual exam. I forgot my marks, but I scored higher in the real one. Of course there could be other factors, like having more time to study or from the trials, we had a taste of what it was like so we were able to improve. We all had the same test, Advanced, Intermediate and Standard.
To get into Extension 1 Maths, I guess you have to do well in Advanced Maths, not just cope. My Maths teacher said you should be getting 90's in Advanced or Mathematics 2 Unit, your teacher should tell you if they think you will be able to do it. There is a big difference between 2 Unit and Extension 1. 2 Unit only tests your ability to remember formulas. Extension 1 tests your ability to use your formulas, sometimes it is hard to recognise what you need to use. It's about practise, if you do heaps, you'll recognise it and know what to do quickly. Time is very limited in Extension 1. There isn't usually enough time to check your answers and sometimes even just finishing the exam is hard. I think it wouldn't be anywhere near as bad doing it without the time limit. There's no pressure from tests you always get and there's no pressure for time.
That's about it I guess. You can always try it and drop down to 2 Unit if you find it too hard. 2 Unit feels easy if you're doing Extension 1 so even if you drop it, your 2 Unit mark should be good so it's alright. Our teachers give harder versions of 2 Unit in Extension 1 which can appear in 2 Unit assessments and exams.