Ah yes, you are indeed right, I forgot about that part, On my board I have a jumper that can switch the I/V resistors between 270 and 120 ohm
Also changing the bias resistor (From pin 6 of the chip(s) to ground changes the characteristics quite a lot, I have a 1K trimmer in this position and eventually settled on about 310 ohm,
Whilst I agree that the chips do get a bit warm I don't know if cooling is needed I forget what the enviromental spec for the device is.
As multiplying the number of devices basically increases signal to noise ratio and increases the current capacity of the output. The signal to noise increase is 3Db every time you double the number of devices, so going from 1 to 2, 2 to 4, 4 to 8 and so on, I think the reason you found the big increase when adding the last two devices was that this was where the impact is present, the benefit is logarithmic not linear.