Trouble is that 5kg may dull his greatest asset, speed! Slater is so much better than most other fb because of his speed.
Yes he is turning into a good defender and is forcing balls lose as well as stopping offloads. One can only hope Moses will be almost as good next year with more training under his belt. Already he has improved tremendously at Parra.
Admittedly, 5kg was an arbitrary figure.
French will never be the greatest kick returner, because he has a lightweight frame.
But as long as we have wingers like Hayne/Semi/Auva'a to cart it up, we are fine.
French has a great footy brain, speed, and class - not to mention a great attitude.
I truly believe he will be a top class fullback, even given his size limitations.
Part of what makes Slater so good is his positioning, and that principally comes down to speed (ability to read the play, and react in time, is a huge factor, too).
I believe French reads the game extremely well, and already has lighting speed on his side.
I'm a massive fan of his.