k-frame strengthening
Welding all the seams does wonders for adding strength to the K-members. I've done it on two that I have now, one for my Duster and one for my EL5 Dart. I did one with a TIG, one with my MIG. Needless to say the MIG welder is a TON faster. But, I'm not powdercoating anything so I wasn't worried about that.
The Firm Feel kit is nice, I bought a "used" one, but Firm Feel still has them. Nice if you haven't done it before because it gives you an idea of what to do. But if you have any fabrication skills it doesn't take long to make up the plates.
The steering box reinforcement is probably the most important add on other than all the seam welding, the mounts have been known to break loose from their factory welds because they flex so much. Boxing that mount really tightens up the steering. Reinforcing the LCA and strut rod mounts helps too I'm sure, but probably not as much of an improvement compared to the seam welding and steering box reinforcement.