There has been one instance I know of on here where someone has switched their 57 for a 54 (you can read back through the posts and find this I think). This decision only transpired after a good long ride if I recall correctly.
This was I. Pretty much spot on. Noticed I was coming forward on the saddle, so obviously overstretched. After a long ride this caused many aches (and i am not old).
This was despite the 57 feeling great at first and on a short tide. Luckily Decathlon exchanged no questions asked. Top service.
The 54 has felt much better, and I will be having my first long ride on the 54 on Thursday, and look forward to it much more. I'm no longer overstretched.
Also, that T3 on ebay above, I much prefer those decals! How do i get some of them?