to be honest, i wont worry about it too much. the important brake lines (ie fronts) will be completely stainless steel already. the ones that arent changed are 2 very short sections in the back. i doubt you will be able to feel much of a difference since the back doesnt do most of the braking anyway. i say just go for the 4 line kit and call it a day.
by the way, when i took out the oem rubber lines, they are Very thick and look to be very durable. they arent just regular brake lines that i've seen in other regular street cars.
Current: 2013 F10 M5 & 2013 GLK350
997.1 GT3RS (gone)
2009 WRX STI (gone)
e92 M3 (gone)