First, as others have pointed out, its not the tank material - its the buoyancy characteristic.
Second, its ALL in the mass of the gas, to the 90th percentile anyway. Nothing more or less. Why, you ask? Here's why:
1. You MUST NOT ditch at depth. Anyone who dives knows this, or at least should know this. IF you ditch weight diving wet you are screwed as you ascend - your suit will decompress, gain buoyancy, and you will be unable to stop the ascent. BAD BAD BAD! Ditching weight is for the SURFACE, where you're up and want to insure you STAY up.
2. If you cannot swim up the rig from the bottom with a total BC failure, you need a second, redundant source of buoyancy. This is usually going to be a drysuit, but for wet divers it could be a lift bag. The point is that you NEED some means of being able to swim up the rig, not to include ditching weight.
3. For "deep divers" (beyond rec depths) this works out pretty well since in general as you go deeper the water gets colder, and your exposure time increases (since you have deco to take care of). As such drysuits and technical diving work well together, even in what would otherwise be "warm" (at the surface) water. You can become hypothermic in 80 degree water, and you can become dangerously so in 70 degree water. Below 70F water dry suit protection for any deco dive is pretty much mandatory simply due to hypothermia issues.
4. You must be able to hold position in the water column at anywhere from the surface to maximum depth at any point in the dive, and it is particularly important to be able to do so without significant exertion when LOW ON GAS, as exerting to stay DOWN increases your gas consumption AND makes you MORE buoyant!
So where does this leave you?
1. You need to do your weight check at the surface with a nearly empty tank. If you can remain in the water column with only breath control at 1' under the surface with an empty tank, your rig is weighted properly. Period. If and only if you are inescapably negative with a nearly-empty tank and zero or very little ditchable weight in your given configuration are you overweighted and/or you need to consider a different tank config.
2. The more gas you take, the more it masses. Thus, the more negative you will be at the START of the dive. The material the tank is made of and its buoyancy, empty or full, do not affect this.
3. If you dive in a place without a hard floor, or where the floor is below the MOD of your mix or where you are willing to decend to, then you must have a secondary means of buoyancy IF your total negative buoyancy at your maximum planned depth, with full tank(s), exceeds your ability to swim up the rig.
4. If you cannot EASILY swim up your rig from planned depth at the "rock bottom" gas point (accounting for any deco obligation you may have incurred) then your exposure protection is being excessively compressed by depth (since at this point most of your gas mass is gone) and you need to consider either redundant buoyancy or whether your exposure protection choice is a wise one. That consideration will likely lead you to choose to dive dry.
In general even a wetsuit with a single LP104 will be just fine, provided you have a lightweight (not significantly negative) rig otherwise. For example, with my BP+Wing (steel BP + STA) I need 6lbs with an AL80 in a 3 mil wetsuit, and 1 or 2 lbs with a HP100.
If I wanted to dive an LP104, I could drop back to an AL BP with no STA, or use a lightweight STA, and still have a balanced rig.
Now DOUBLE LP104s wouldn't work in that configuration.
But double HP100s would!
Finally, that picture of MHK is pretty funny. Here's the guy who is one of the "poster children" for the GUE "must be fit" crowd - and he has a pretty obvious beer belly WAY beyond what I had a year ago in that wetsuit....
Amazing how the double-standard works...
