no need to do the rears.
top of the line jdm s14 and s15 have something nearly identical to z32 front, stock usdm s14 rear brakes. it works. you never want your rear locking up before the fronts anyways, and when you mess with the bias to "balance" the rear more, you run the danger of locking up the rears before the fronts when going through a turn. a danger which is very real when daily driving and seeing unexpected situations come up.
now if you were driving on a track and will be doing most of your braking in a straight line, trail braking into a turn while completely expecting what will happen, etc, then go ahead and give the rears some more stopping power.
As far as "stopping power" is concerned, the limit is your tires, and as long as you can lock up all 4 wheels, you dont need any stronger calipers. if you want better initial bite, get some good pads. axxis/pbr ultimates are cheap and aggressive as hell. also doesn't fade easily at all.
now if you need to slow down very aggressively very often such that the ultimates fade on you... then you need the z32 brake upgrade.
your rears needing replacement so soon... check the adjustment for your ebrake. you may be dragging it. or your front lines may be kinked/not bled. in either case, this should be your PRIORITY, since rears locking up before rears is never a good thing unless you're trying to ebrake drift.
now i DD and lightly autoX my s14, and i have the z32 front brakes...
i'm one of those types that can't spend money on simply OEM replacement, and i liked how the bigger brakes looked. that is my honest to god reason for putting them on. boo, i'm a ricer
you'll have to decide if you'll eventually need the capability or want it, and such and such.
i love my brakes, they do work well.