Slow response from brakes at higher speeds

I'm having a brake issue with my sister's 2004
Volvo S40 1.9T with approx. 230,000 miles on it. I
changed the front and rear brake pads & rotors.
Soon after she told me when she was traveling at
highway speeds and had to come to a quick stop,
the brakes would feel less responsive. They also
required more effort to slow. This only happens at
higher speeds (50mph+) and when the vehicle
comes to a quick stop (CA traffic). Orginally this
car had very responsive brakes especially at slower
speeds. I did eventually change the front calipers
because I noticed uneven brake wear on the front
rotors. The outside edge of the rotors were worn
but you could still see the crosshatch marks on the
inner portion. She did say they felt a little better
after the caliper replacement, bleed, and additional
break-in time. I'm debating whether to replace the
rear calipers too due to rust (Originally a New York
car), but I'm not sure if this is also causing her
slow responsive brakes since the fronts do most of
the work anyways. I'm also thinking about possibly
a brake booster issue, but from what I read most
symptoms happen at slow speeds. Any
suggestions and ideas are greatly appreciated.
Thanks!