I mean I don't particularly think that revising from Year 9 is too early. I mean I'm in Year 9 as well and I do revise ahead sometimes and I guess it is somewhat good. The pros of practising now is that you're going to know everything you need for GCSE before everyone else and potentially have more time to practise on it. The cons of it however are that you might be so early ahead that you might not feel the need to listen in classes. To be honest if you want to 'practise GCSEs' then either invest in some good textbooks or look at the syllabus of your subject on the website of your exam board and research online. I know that investing in the new textbooks that I have now have helped me a lot. I will usually read through a topic in a unit (or the entire unit if there's not a lot), then watch videos about it, then make notes and read them and then have print out practise papers on it once I feel comfortable.
Also, it's 2% of the people who have taken their GCSEs who have achieved a Grade 9, not 2 people.
(Sorry if this was really long and didn't help)