I made an 6" extension for my three point lever way back when I first got my tractor. Mine isn't nearly as nice as yours Pedro, but it has worked over the years and does help. My problem was in fine control of the height of the box blade when grading. With the OEM short lever it was just too hard to raise or lower the blade just a fraction of an inch instead of one or two inches. I still have to be careful as it is still touchy in that regards, but the longer lever has helped considerably.
Your adjustable stop, K5lwq is another good modification. I have considered doing that but just haven't gotten around to it yet.
I find it amazing that my 60 year old Ferguson has a 3 point lever that is very well designed, with index marks that actually work and are repeatable, and easy to use adjustable stops for both up and down. That lever is so much better designed and practical than any I have seen on any modern tractor.