Others will disagree, but a shock that is truly "bad" is leaking. You're not gonna get the same effect bouncing it like a car. So most folks just replace them once they get some miles on them (and can afford new ones) Theres' no doubt that Ohlins or Wilbers will make a dramatic difference in feel, even with new OEM shocks.
So, if it were me, I would replace the stockers at 20-30k miles and re-install
them when ready to sell the bike. I've heard nothing but good about Wilbers
shocks and they are cheaper than ohlins. YMMV!