Okay, I'm currently diving a SS back-plate. Basic Halcyon Eclipse. I want a lighter option for warm water. Aluminum back-plate like the one from HOG looks like the best bet for me
I have a Halcyon alloy BP, no problems. Also have a few SS plates
Like-for-like (same size, design - excluding skeleton plates etc), the saving is only about 3#... to me that doesn't make much difference to my packing weight. In water, I can wear the alloy plate with a 3mm shorty (or less) & an AL80 without adding any extra weight, but I can also comfortably wear an SS plate without the extra 3# bothering me. If I'm wearing a 3mm steamer or thicker (temperate waters <26C), I need to add weight, which kinda defeats the purpose of using alloy in the first place (except you can just borrow a weight instead of lugging it around with you, but still only 3#ish)
In summary, you need to decide if shaving 3# (like-for-like SS vs alloy) from your travel or dive weight is really a worthwhile benefit. In hindsight for me it is not, and if people ask I would generally suggest they just go with SS (especially if, like you, they already have one)
YMMV