No change to the shock is necessary. Even with the completely stock shock the improvements will be very noticeable.
Both can work well with this link. We prefer the coil for rougher trails with lots of roots and rocks and air for trails that are smoother but with big hits. There’s a middle ground of course and that’s where it’s more of a toss up. If you already have something installed on your bike and don’t feel like replacing it there certainly isn’t any reason to.
This link delivers substantially more bottom out resistance than the stock link. Using 30% sag should be your starting point. If you feel you need a little more first try adding a few clicks of damping to account for the change in leverage curve. If you still want even more you can try going up 25-50 lbs in spring rate or 10-20 psi in pressure.
At 30% sag this link will also deliver much better small bump sensitivity so start there. If you still feel like you’d like a little more try running less damping. The link makes it so the suspension doesn’t rely on damping as much to avoid bottom outs so running less isn’t an issue.
The link doesn’t change the static geometry of the bike. The only geometry affected is wheel position at full bottom out.
Your best bet is to give Specialized a call and ask them directly. Only they can make that call.