2.5 kg of that will be the weight of the steel BP. The disputed assertion is that a typical plastic/fabric BCD is buoyant on its own account and so the change will allow for less weight overall, including the plate. My experience is that any advantage is marginal if it exists at all. On the other hand, BPW have genuine advantages of simplicity, strength and so forth, and I prefer moving weight off the belt to the backplate. Nothing magic though.It's a 3 kilo of lead difference for me between my no-padding steel BP with small wing and a jacket BC.