Speculation is ultimately driven by scarcity. There's no purpose in speculating if you can't generate an arbitrage. Greater productivity on the part of small farmers will alleviate scarcity, driving food prices down.
Similarly, there are lots of markets that are unserved by the global food chain. How can one say that food subsidies in developed countries can affect these unserved markets? They can't sell to the developed world, and developed world food doesn't really make it there.
I spent some time in Colombia, not quite a developed or a developing nation, in mostly big cities, and even there much of the food consumption was local. Sure, bigger farmers would be able to sell at a profit to Western conglomerates, and Western food was available (at steep prices) in the large chain grocery stores.
But most people shopped at the ubiquitous small family-run groceries that served a mix of local agricultural output and mass-market, also locally-produced goods. I'd be very surprised if Western agricultural innovations weren't being employed to grow all of the great-tasting produce I ate there. Seeds are much much easier to transport than mangoes.