I agree with your math, but not your analysis.
In the late game, both Cull and Doran's Blade are going to be sold. It's not uncommon to sell Doran's Blade well before starting a 6th item so that you can hold multiple components of a 3rd, 4th, or 5th item, as well as potions or a pink ward.
This means that the stat advantage is only active during the period between the selling of Cull and the selling of Doran's Blade, which also means that you actually have 580 gold that your opponent doesn't have. Doran's Blade gives a total of around 600 gold in stats, so whether the Cull user is ahead in the mid game depends on what the 580 gold is spent on.
Rapid Firecannon has a combine cost of 550 gold. The value of the passive is difficult to evaluate, but I'd much rather have a Rapid Firecannon than a Kircheis Shard, a Zeal, and a Doran's Blade.
Youmuu's Ghostblade has a combine cost of 1000 gold, so it's possible to be 580 gold ahead of your lane opponent and not be able to complete it. In this case, I'd rather have Caulfield's Warhammer, Serrated Dirk, and Doran's Blade than just the Warhammer and Dirk.
In the end, it depends on timing. In the mid game, it's possible for the Doran's Blade user to be stronger in lane/teamfights because of the extra stats, but it's also possible that the 580 gold from selling a completed Cull is enough to finish an item and the Cull user is stronger in lane/teamfights.
In the end game, both Doran's Blade and Cull are sold, but assuming everything else is equal, the Cull user has 400 extra gold over the Doran's Blade user.