Mate go to Subaru and buy the proper bolts, they are a rubber gromet with screws going into them to make them expand, much less painful when you nudge something - instead of pulling the screws out of the bumper and lip, the rubber gromet gives and can be put back in without any damage, I was pleasantly surprised when wifey nudged a gutter, expecting the worst (busted lip etc) and all that happened was the lip deflected out of the way and the gromets came out.
Al