First off, I stated that I was explicitly discussing GWM using the -5/+10 versus GWM not using it. You appear to have missed that in your response here. So if we are not discussing the same thing, of course there will be miscommunications.
I do absolutely think that GWM has the potential to do huge amounts of damage. But I also think that your "hard data" here is skewed and you don't even see it. Your group purposely buffed this PC with Fly and Bless (although other PCs were also buffed with Bless) and (eventually) Foresight and advantage from familiars and such, and you wrote down the damage results.
But this is anecdotal. You have stated before that your DM allows the PCs to dictate combat and such. With a different DM, this PC might not shine quite as much. For example, a DM who throws large groups of foes at the PCs could easily grapple/prone this PC, giving him disadvantage (or having NPCs put up Darkness or Fog Cloud, or using traps to split up the party, or killing familiars, or a wide variety of other things that can shake up the effectiveness of a party).
Another problem with this is that you put all of this effort into making this one PC shine so much that you totally disregard how much other PCs can shine if the spotlight is focused on them and the party tactics revolve around them instead.
As an example, at level 6 when a fire Draconic Sorcerer gets Elemental Affinity, if he has Elemental Adept fire, he does 36.3 average points of damage on average with an Empowered Fireball spell instead of the normal 28 (+3 more for Charisma, +5.3 more for re-rolling 3 bad rolls and all 1s automatically becoming 2s). At level 12 with a 20 Cha, this increases to 39.3 (5 rerolls, +5 for Charisma).
Many encounters, he should be able to get 3 foes in each of his Fireball spells (sometimes more foes, sometimes less foes, but usually at least 3 because he casts the spell when he has the best chance of hitting more foes). At level 6, that's a range of damage of 163 to 327 points of damage. Typically at these levels, foes have a Dex mod of +1 or less, so average damage here against DC 14 or 261 average points of damage per day, 87 in the round he casts (29 per foe). The 6th level GWM fighter in this party averages 28 or so points of damage per round (2+ attacks, assume 60% chance to hit at -5/+10 due to buffs like Bless or Feinting), so it typically takes the fighter 3 rounds to do the same damage that the Sorcerer does in 1 round.
The Sorcerer then has other options to add even more damage in the other 2 rounds. Granted, at level 6, this is only 3 times per day, but that's 3 encounters a day where the Sorcerer is typically going to do more damage than the Fighter (and the Sorcerer still has 7 more spells he can use). Granted, there will be times when the Fighter rolls huge damage, criticals, etc. But there will also be rounds where the Sorcerer rolls 5 or better on most of his damage dice and/or he gets 5 foes in the blast and/or most or all of his foes fail their save. Sure, the Fighter might nova at 75 points in a single round, but the Sorcerer might nova at 200 points.
At level 17 when the GWM Fighter is near his peak often doing 75 or more points of damage per round (a portion of this because he is buffed), the Sorcerer gets upwards of 12 Fireballs a day (potentially 2 or 3 per encounter) ranging from 19 to 65 damage per spell per foe (limited in number to more powerful spells) or 692 to 1384 points of damage per day with a save DC of 17 (close to 1200 points of damage). The Sorcerer can still average better damage per round as the Fighter for those 12 rounds (more if he can get more than 3 foes in a blast, the times for which he will use higher level slots). The Sorcerer also has a lot of other spells and options.
Granted, this tactic does not work against creatures immune to fire. And if the party plays dumb, the Sorcerer will not be able to get as many foes in without hitting allies. And he will not be able to use it every single encounter. And the Fighter has more opportunities to wipe out creatures at the higher levels and will be doing more damage in rounds when the Sorcerer is not casting Fireball. But this is a lot of levels where the Sorcerer does comparable damage per day than the Fighter. No doubt, the Fighter does more damage at higher levels, especially if the rest of the party buffs him.
But if the party tried to assist the Sorcerer (setting up a front line so that most of the foes are on one side of the fight, pushing foes into Wall of Fire spells, funneling foes into chokepoints, etc.) like you have the party assisting the Fighter, the Sorcerer would shine too. The numbers above do not take into account any other ways to optimize this Sorcerer or to have other PCs buff him.
I'm not an optimizer and I was able to find a PC who could do comparable damage to a GWM fighter for many levels (maybe not at level 20, but probably just as good per day at levels 6 through at least 15 or so, a big chunk of the levels). I'm sure that someone who really lives and breathes optimization could find other combinations that work even better, even for melee types.
