Yes, rubber lines “swell” causing a soft(er) pedal. While this can happen in older cars, this is not the primary gain from upgraded brake lines. Yes, the Teflon tube wrapped with braided steel has a much lower swell than rubber and textile lines. The firmer pedal that is immediately apparent is mostly from the smaller
internal diameter of the Teflon lines. Since the Teflon is more rigid, you can get away with a smaller diameter without increasing the risk of collapse. This means that you actually need to move less brake fluid to achieve the same application of brakes. The result? A more responsive pedal with greater feedback.
Having said that, (and here's my disclaimer/caveat), I would NOT run SS braided lines UNLESS you *routinely* change them annually or bi-annually. See below for more info (and this what many of us who have toys we race with adhere to
religiously):
Brake Lines - NSX-Wiki.
Lastly, if you have a soft pedal, there are other just as likely culprits, like leaks, air in the system, a failing brake master cylinder, etc, that you should be looking into as well.
Just my $0.02.