I reckon 4 years on the rears is fair enough for a change of pads, regardless of mileage. At least the sliders can be cleaned and relubed, and the piston can be retracted, which in my opinion will make everything less likely to seize /stick. The garage said 15% meat left on them so what's all the fuss about?
If someone knows a lot about a 3.5 focus St and can definitively say what effect traction control and cruise control have in the rears, and factor in OP's use of the car, maybe they can give an informed opinion, but simply comparing the rear pad wear of completely different cars is pointless.