IMO, if you are satisfied with your car, except for the ride, get stiffness adjustable coilovers.
HKS just came out with the HiperMax LS Plus coilovers and these cost about USD1300 - 1500. The good thing with these coilovers is that these are optimized for comfort and you can adjust both the ride height and stiffness. If you just change the springs, you might think that the SE is too soft.
I got Apexi WS coilovers (also comfort-tuned) myself with 18s rims. I do not think that these are much stiffer than the original springs/shocks. I live in Hong Kong where the road conditions sometimes are very poor and think that the combination Apexi WS/18s is ideal for my GS300.
Alex