I did L2 further maths a couple of years ago and while I agree that there's not much benefit to doing it, I enjoyed learning the extra content.
From what I remember, there's an overlap between the higher GCSE content and the L2 FM content, especially in the circle theorems topic. If you're in Year 10 I'd say focus on getting to a solid 8/9 in GCSE maths first as that will give you a good foundation to tackle L2 further maths. After that point, it's just doing practice questions by topic & past papers.
About 70% of my current A level maths class did L2 FM and they found the transition to A level maths easier than those that did not, especially in calculus. I'm also doing FM A level so I'm glad I learned a bit about matrices through L2 FM