if you use the wagon leaves to make a custom pack (2x leaf packs = 7 leaves and 220lb) then it will stiffen up the arse. alot. try perhaps adding one leaf (i'd recommend the second from bottom) and see how that goes. you can get custom leaves made up for usually around 30 bucks apiece
any more than one leaf and you'll need custom shackles, which need to be joined to the spring. clamp shackles are illegal for street use
the slapper bar isn't good for handling, jonny rochester tried this in his te37 replica, and said they come on too hard too fast and fuck everything up, stock is better, a well designed gradual leaf pack is even better. the slapper is made so you can stuff a heap of heavy crap into the back of your corolla wagon and drive off without bottoming out (very slowly with your stock 3k)
Where do you get custom leaves for $30?
I've been told it would be in the hundreds.