The rear suspension on new-gen is much improved over last generation due to some changes:
1. shock with stiffer spring and damping along with preload-adjustment
2. revised leverage-ratios with knuckle!!! This alone probably accounts for 80% of improvements. New knuckle moves shock
more for same amount of wheel-travel. This increases leverage shock has on swingarm and same shock would have stronger "push" on swingarm.
When I upgraded my pre-gen to new-gen rear-suspension with entire swingarm assembly, for giggles, I re-installed original pre-gen shock. Wow, it actually worked better!!!
So rear-end on new-gen bikes don't need as much upgrading as fork.
For fork-springs, other brands like Progressive or Bitubo can be had for less on eBay. No-cost option is to cut off 4" of spring and replace with PVC spacer. Back in '80s, when aftermarket products wasn't as abundant, this was pretty standard procedure.