I have upgraded brake lines to install too. Think they really make much of a difference?
From past experiences the pedal modulation is much more direct. Less expansion under medium to heavy braking. Didnt really feel anything at the initial bite but i was also running some pretty extreme pads on the car at the time(my 05 GT).
On the Fiesta i havent really had an issue with the stock lines even with the Hawk HPS pads(which bite significantly less than the OE performance pads). The Hawk pads are great once warm but dont offer that instant bite that the factory pads offer. I will probably swap back to the factory pads with the new lines when the time comes. No rush since the factory lines havent shown any issues.