I'm getting confused here - IE800's full stock cable got 2 parts, let's call the lower part "3.5mm cable" and upper part (i.e. linked to earphones) "2.5mm cable".
Now if I'm understanding from you correctly, 2.5mm cable is 4-wire, 3.5mm cable is 3-wire. Then isn't a balanced cable with 2.5mm both male and female plug all we need? Why is any soldering neccessary?
The 2.5 cable starts out with 2 wires from each of the two channels, left and right: 4 wires in total. However, the stock 2.5 cable is terminated with a 3-pole plug. That means two of the wires are soldered to one common pole. For that reason, in order to create a balanced connection, you need to cut the stock plug and resolder the wires to a 4-pole plug, each wire connected to its own pole: 4 wires, 4 poles.
Whether a cable is balanced or unbalanced has nothing to do with whether it is 2.5 or 3.5mm. You could have jacks or plugs in either with 3 or 4 wire connections.