The selection process is very competitive. You have to identify your biggest weaknesses and concentrate on those. If you believe spelling is going to hold you back the most, and repeating English will improve that, then go for it. You will be required to write a number of essays throughout the selection process. I believe both those courses are based on finding those weaknesses and ensuring the initial stages of the selection process aren't a complete suprise, ie you've done an aptitude test before doing the RAAF one so know what to expect.
If you think getting 92% instead of 84% will make you look that much better, don't bother - you need to be competitive against other applicants in all areas. At the end of the day if you don't fit the personality profile they're looking for, you won't get too far. If you do meet the personality profile, minor differences in your English score won't affect your chances, your overall score throughout their selection process is what matters (competitiveness is largely based on scores from flight screening and the final selection board interview).