Worth adding foam if I'm replacing cladding?

In short, yes it is unless you are talking about that fanfold stuff. For best results you should consider two layers with offset seams as all materials do expand & contract. Not only will this decrease your heating & cooling loads but also help reduce air leakage & noise transmittance (if done properly). One other item to consider is utilizing rain screen technology where you leave a gap between the foam & siding allowing things to dry out For more on this - http://thehtrc.com/2013/common-sense-weather-resistive-barrier