I've known a few people say RC springs are very soft
Don't think its been mentioned but putting the rear springs from the diesel might be a cheap and effective way to stiffen and lift a couple of inches.As Phil said rear shackles replacement will help as well. But an extra 2" length shackle will only lift 1" ........and tall shackles get wobbly and look so 80's so a max of 5" eye to eye is really all you want......and is the length supplied by RE and the like.
I could do to improve performance - I noticed the other day whilst chasing a big diesel Beemer up a hill that it's feeling slightly down on power. It didn't really want to go above 70, then couldn't even hold that speed, when I know it can usually get 85 up there. I'm guessing my air filter probably needs replacing - it definitely has visible mud on it.