David,I have been thinking about this for a while now...
If your car has external bumpstops to limit suspension travel, then heavy braking should not bottom out the shock absorbers at all, as the suspension should collapse onto the bump stop.
If your front coilovers have internal bumpstops, then the shock absorber should also not bottom out (it should collapse the bumpstop). If a strut/coilover has no internal bumpstop, the shock absorber typically is long enough that you won't bottom it out either (otherwise you would damage the valve stack inside or rip the housing) - you should be experiencing coil bind first.
However...whether you have bumpstops in your struts or not, the force going into the lower strut bushing is always the same, since the weight of the car and the kinetic force remains the same.. Therefore changing the shock absorber or the spring is not going to improve the life of the bushing at the bottom of the coilover.