Dude, that website is so misleading regarding which models have LSD's it's frigging ridiculous!!
Go here and review the car specs by year:
http://www.cars101.com/impreza.html
As for the LSD, I had one in the 2001 Forester at the same time as I had the '98 RS without one, I can honestly say I didn't notice any difference, and that was in rain, snow, sleet.....