Unless a shock is damaged severely, you normally would rebuild your shocks rather than replace them. There are a number of shops around that can rebuild them back better than new with improved valving, new seals etc. Switching the springs to a dual rate setup will also improve the performance greatly over stock. Shock rebuild/service intervals can vary greatly depending on what kind of terrain you ride and how hard you ride. Walker Evans shocks on an XP1K under average trail riding conditions would probably go 1500 to 3000+ miles before needing a rebuild. YMMV. I think most shock manufacturers recommend a rebuild at 1500 miles. If your shocks are leaking anywhere, they definitely need to be rebuilt.
You will get many opinions on tire pressure. Personally I run mine at 14psi most of the year, 2 psi under what the manual says. Anything under 12psi and your chances of tearing a sidewall go up quite a bit. And the lower you go, the more likely it is you will tear or get a pinch flat in the sidewall. Yes it rides softer with lower tire pressure but I'd rather deal with a little harsher ride and not have to replace a tire or worse a bent rim.