If it helps, on twin 12s, 3mm steel backplate and a neoprene drysuit (Othree PBB undersuit) I need around 12lbs (5.5kgs, google says) of lead to stay down comfortably though I think I could possibly lose a little myself.
is it a compressed neo suit?
That sounds like a lot to me Gledders did a weight check with me and reackons I'm 2-3kg heavy with the same set up as you except I don't have any wieght!
I'm not the smallest person around I had to argue to get an HSE medical as I'm officioly clinicaly obease, I need to be 3 inches taller for my wieght(note I need to get taller not lose wieght).
I would suggest doing a proper wieght check you'd be suprised how much you can drop