I don't believe it is just being ahead that makes geddon auto-win. Pairity or just below it also doesn't make it a game winning card. You have two creatures on board, your opponent have one sized about your biggest creature and suddenly geddon is uncastable. You play it against a control opponent with mana open then eats removal just to see your opponent untaps and plays whatever and gedon didn't win you the game as well. Both of these situations surfaced in the past two or so months in my cube. I have seen gedon take game one, but never the whole match.
What situations have you encountered that made geddon win the game by itself?
I am not sure I can comment on white aggro underperforming without geddon because monocoloured decks are almost never played in my cube. I think the problem lies with cubes having too many efficient card advantage spells to play in the early to mid game that makes racing a losing strategy. Some cards that I have seen make white aggro better:
Hokori and
Thalia(I know, it's land denial as well, probably shouldn't mention),
Hero of Bladehold,
Ajani Goldmane,
Elspeth,
Accorder Paladin,
Mikaeus and
Mirror Entity.
(Disclaimer: I'm not trying to bully people into playing armageddon or something, I just want to pull the discussion to a point where we can safely say that we understand what makes cards oppressive and what makes them being fun. I know that I am kind of in a weird position to say anything about oppressive cards since I play with a powered list, but my friends love to play with abusive over-powered cards, and I believe my job running the cube is to try and make sure that these cards are played in a context that they can be as much fun to everyone as possible.)