If you read the instructions, you may notice that it's strongly recommended to take IELTS unless your first language is English. If you decide to ignore that and go with a written submission anyway, it will be completely up to the immigration officer if they give you any points.
Loads of people have studied/graduated from univerities in English speaking countries even though their English isn't the best. Loads of people have lived and worked in English speaking countries even though their English isn't the best. Loads of people have tried to trick CIC into thinking that their English is very good because they have studied or worked in an English speaking country for a while. That's why CIC wants you to take IELTS. If your English is as good as you think it is, it should be a breeze and nobody will be able to refute your points. If you go with a written submission, it's your risk to take. Up to you.