You are ignoring the point that the doubles diver has the edge in terms of reserve on dive one and gives up little even with small doubles on dive two and retains the edge on dive two if they are smart about their selection of doubles.
Why would a doubles diver invest in double AL 80's when double 100's weigh the same? Why handicap them with double 80's that virtually no doubles divers use? Just to keep the balance sheet in your favor? No. Lets not get hung up on apples to apples and instead focus on the real world.
And you have to admit the average pony diver is not usually using a 30 - more like a 19 or 13, so you are in effect trying to give the edge to the single tank diver already with a superior sized but larger than most real life situations pony.
The weight difference between double HP 100's and an 80 with a 30 cu ft pony is about 20 lbs once all is said and done, so the weight penalty is not as bad as many would assume in your chosen single tank configuration.
I can also tell you from experience that when you are in the water with a single tank diver with pony and things go wrong at 100 feet or so (poor viz, can't find the up line, don't want to drift away from the boat, etc) that single tank diver (who is even someone else's buddy) ends up sticking to me in my doubles like glue. It is a case of the "you can never have too much gas" philosophy trumping just about every other philosophy when things go south and your AL 80 and pony start looking small.
And as pointed out in the post above, doubles offer a degree of flexibility and capability that cannot be approached with a single and pony. I know - over the last 25 years I have moved from a single to a single and small pony to a single and large pony to independent doubes to manifolded doubles, as each step offerred more safety, flexibility and capability than the previous step with minimal weight penalties when you consider the advantages gained. It is just a normal progression and if you are not there yet, thats fine, just don't get the mistaken impression that you are going to be able to add any elightenment to those who are already there.
Larry