On the WRX at temps below about 35-40 degrees the asphalt sound deadner just pops off in big flakes. At moderate temps it is a PITA. If you get it hot enough with a heat gun it gets soft enough to be scraped off.
Pretty much a choice of evils, Do you mind working in a cool/cold environment, if not go that way. Get a medium sized hammer and when it is cold just tap the surface of the sound deadner VERY lightly, going over the whole surface. You can hear it popping and crackling. then just take a stiff putty knife or a gasket scraper and it pops off in chips about the size of large potato chips. Process goes very fast at freezing temps.
I don't like the hot technique because, there is a small fire hazard, the fumes bother me if you get things a little too hot, and its harder to scrape it off clean where when cold it leaves a nearly clean surface.
At moderate temps the stuff is pretty tough and a little sticky, don't even try to do it at normal temps you will hate your self.
Just removing bolt ons and things like mentioned on the oakos site, I've taken off about 175 lbs. No extreme mods yet, like bumper beams, racing seats, stock weight clutch, and still full stock exhaust.
Car weight ready to run, 1/4 tank of fuel, with driver, 3050, show room stock same configuration about 3280 IIRC.