I had the Pro-kits on my car, and they didn't match well with the stock dampers. They were a little bouncy/floaty at the limit. Had the Steeda springs been available when I got the Pro-kits, I would've chosen the Steedas.
Give Steeda a call, I'm sure they'll be able to answer any questions you might have. I'm fairly certain that they are, in fact, linear-rate springs.
Progressive = variable spring rate, depending on spring compression
Linear = consistent spring rate throughout the suspension travel
Progressive springs often ride rougher and bounce more, which will drastically shorten the life of most dampers. Also, at the limit, they often can exhibit unpredictable handling characteristics (snap oversteer), since the spring rate exponentially increases with travel. The increased spring rate can induce an unexpected loss of traction.
Joined the dark side.