Try to change this field to 400 then back to 200, then to 300 and then to 350. You will see, while you are messing with this field, that transfers start to accelerate.
Apparently, changing one time isn't enough. The more you change, the more it accelerates. But as far as I tried this, the limit appears to be four to five changes (in my machine) using values not over 400. Changing it too much, or using high values, can saturate the Mac OS' web connection socket and you will (probably) end up without a web connection and will have to restart the machine.
[robg adds: I tried this with a folder of 256 test documents, and I didn't notice any change at all as I played with the connections setting. If others can confirm or deny, it'd be interesting to try to figure out where/when this trick may work.]

