But my thought is that if there was a global ban on agriculture subsidies,

There really does not need to be a "ban" perse.. and it would be impossible to implement on a global scale with sovereign nations being what they are... well sovereign. We can just stop handing them out on the domestic front to anyone other than some uber rural developing/impoverished area farmers. The big farming corporations can already compete on the basis of economies of scale alone.

Outside of that, if the US can produce corn cheaper than anyone else on the planet and enough for everyone to consume.. so be it, every region and market will likely have an agricultural product or commodity which it can produce cheaper than others once properly developed. Now, sovereign nations really do need to maintain semblance of their own independent food security and with regard to that domestic grain production on some scale should always somewhat supported on the level of governments. However, that should only be on the level where the nation is not wholly dependent on imports for all of its food and to a degree whereby if need be if/once foreign imports become too expensive or stop they can rollover and expand on domestic production to meet needs.

Our protectionism (and everyone elses, like China) costs developing countries millions and millions of dollars per country (depending on the crop and country, obviously).

To a degree, however there are tons of agricultural products which impoverished and developing nations could produce to export instead of attempting to compete with grain production from industrialized nations. Haiti as an example, with regard to say luxury and cash crops, if the production base was focused on those products, expanded on and distribution systems improved on they could easily be among the top producers of those goods in the world. However, instead of expanding on the production of luxury and cash crops the Haitian government has a history of implementing excessive tax schemes on specific promising ones that in the end hamper and undermine overall production instead of helping promote growth in the nation.

So, while the subsidies can have a negative effect... the total impact is hardly wholly dependent on them. Even if the subsidies were not there smaller less developed nations would not be able to compete with US and various European nations on productivity of various grains. This is even without going in to the impact technology has on productivity... cost of production and totqal output on a 2-5 acre family farm with people using cythes to cut down grain vs a 10000 acre industrial one with a semi automated grain mower thingy that does everything a thousand times faster.... (drunk ESL kicking in I forger the word for those...)