*waves*
My Geoffrey has lost 1kg in the past 8 months. Contrary to some other suggestions, I was feeding nature's menu/bozita and swapped to the Royal Canin Ultra light pouches and light biscuits. To start with, I gave him one pouch each, morning and evening and half a pouch at lunchtime and a small pinch of biscuits at bedtime. As he lost weight and I wanted to stabilise him I started swapping the lunchtime half pouch into a small pinch of biscuits, and he is now nicely stable at 6.3kg - which still sounds a lot but my vet says he is a big cat and it's about the right weight. I was already playing with him a lot, but the other thing I started doing was removing Milly's food when she walked away from it - previously he was just hoovering up her leftovers so I was quite strict to stop him doing that.
So I guess although RC may not 'technically' be the best food, having tried lots of options over the past couple of years, it's the thing that's worked best for me! Good luck with yours...it does seem painstakingly slow, but I'm sure you'll get there in the end.