@macado where are you getting those numbers from? That is only true with the Worthington LP85's against normal HP100's or the newest FX100's *which are closer to 120's than 100's* from Faber against their LP85's. "normal" HP100's are 2" shorter and bigger around than LP85's, but anyway, that's neither here nor there.
To the OP, your configuration is not a safe diving configuration. I'll just go straight to the point, and here is why. 7mm suits have no business doing deep diving due to the suit compression, especially if it is a farmer john. You also shouldn't dive tanks that are that big in a wetsuit because at depth the odds of you being able to kick them up from the bottom in the event of a wing failure are basically 0. Compound that with the fact that a steel backplate will grossly overweight you to the point that it is idiotic, you're asking for trouble. Your wing has to do 2 functions. Hold the rig at the surface, and compensate for buoyancy. Let's do some math.
Bands-4lbs
Manifold/valves-6lbs
Backplate-6lbs
Regulators-4lbs
Weight of gas-roughly 16lbs
Buoyancy of tanks-anywhere from -7 with Worthington HP100's to 0 with LP85's
You are at a minimum of -36, likely closer to -40lbs at the surface.
Wetsuits have a buoyancy compensation of roughly 3:1. I.e. 1lb of wetsuit at the surface requires about 3lbs of lead to sink. 7mm's are roughly 5-7lbs for most people, so 15-20lbs of lead to get it to sink. You are now at a minimum of 51lbs, potentially up to 60lbs lift requirement out of your wing. You are starting the dive naturally overweight because the the ballast of the empty rig exceeds the requirements of your exposure protection unless you're diving LP85's in a 7mm farmer john, in which case it is still not a safe configuration due to the buoyancy swing. IF you're going to do this anyway, at least do it with a neutral or close to it plate instead of stainless steel, especially if you go with HP100's where you are going to be close to 5lbs overweighted even with a kydex/aluminum plate.
Also remember your exposure. You define cold water, but don't say how cold. 100ft with HP100's on a normal diver, in open water, will net you well over an hour of bottom time, which at depth in a 7mm means you will get COLD, and quite possibly dangerously cold increasing your likelihood of getting DCS. Drysuit in deep cold water diving is a safety concern so please do yourself a favor and consider purchasing one....