Well, I ended up getting the shocks revalved. After sending up all 4 to Autolign in Auckland, turns out only the front shocks can be rebuilt (and revalved). Apparently my rear shocks were OK but my fronts needed rebuilding.
After getting the fronts rebuilt and revalved to soften and the car back on the road, my experience was that "it made a small difference but certainly not worth what I paid" (ended up costing me around $1200 all-up including labour, wheel alignment etc).