If your shocks have bladders in them- the silicone things that you stick the shock cap over -then your shock shaft should rebound to around half of its full length. This in itself is a tuning option. HPI has a firmer bladder for their shocks that allows the shock to rebound slightly quicker after being compressed.
Keep those shafts smooth. When you rebuild your shocks and you see that when you pour out the oil, and notice is not of the same color as it was when you first began, then replacing the seals is the proper response. This shows that dirt is entering the shock body through the orings, via the shock shaft.