Mine is the Stampede 4x4 and the only thing I have upgraded is I added a aluminum drive shaft and aluminum shock caps. I usually don't upgrade unless absolutely needed.
I would try TNTs suggestions and go from there.
Honestly to me it sounds like you have them where you want them. If you go stiffer, your rig won't perform well on smaller jumps. It will be too stiff and bounce all over the place. Chassis slap is pretty much normal so I would leave it just the way it is.
Just my opinion mind you and its what...