Jamis should have a recommendation for the "sag" of your rear shock for that specific frame. See what they suggest and start there.... IME, a shock that is too stiff can make the bike feel like it's trying to throw you over the bars on descents, which isn't a good feeling. An imbalance between the fork and the shock can have a similar feeling as well. Setting the sag on air forks and air shocks is something that anyone can do...no mechanic needed! It's a bunch of trial and error to find what feels right for your riding style and trails.

I really like ESI Chunky grips. They're not ergonomic, which means better grip on the bars for me, but they are very grippy and dampen vibration well.

Can you put the same saddle that's on your road bike on your mountain bike as well (assuming that you like it!)??