Perhaps copper buses are not the reason that battery builders see less voltage sag when using the same number and configuration of cells, but with nickel. But lets look at that for a minute.
The voltage has to pass through the series buses. Lets imagine that it is 0.20 thick pure nickel (7mm wide), very common. In a 14S pack, the voltage must travel across 15 of these strips to get from the positive post of the pack to the negative post. If there is roughly one inch from the tip of one cell to the tip of the next cell, that's roughly 15 inches of nickel. There's no way of getting around having 15 inches of conductive material in that configuration, and it can be nickel, copper, or something else.
Try this...take your battery pack, measure how much actual length is in-between each cell on the bus material, then add it up. Lets be generous and say it's only 12-inches of nickel. Put the test battery under load with a $20 watt-meter to see how much difference there is between resting voltage and max draw (voltage sag). Now add a 12-inch strip of nickel between the existing positive pack connector pin, and the watt meter. 0.20mm thick by 7mm wide.
You have now precisely doubled the amount of nickel that the battery pack is forced to operate under. Put that pack under the same identical load, and see if the voltage sag is the same, or if it is worse. Record the difference.
Now remove the 12-inches of nickel strip, and add 12-inches of 0.20mm thick copper (7mm wide) between the pack and the watt-meter. Put the pack under an identical load just as before, and record the voltage sag, plus...feel if the strip is getting as warm as the nickel strip was becoming.
I am certain that if you try this, you will get four times the voltage sag when adding 12 inches of nickel, compared to adding 12 inches of copper.