keep it, or find a replacement at a junkyard. virtually any older impreza or Forester will have this part. hit the fasteners with PB Blaster, let em soak, and the bolts will come out OK even if they look rusted to crap.
that shield is primarilly about heat protection of the driveshaft...unless you WANT to cook the rubber mounts and U-joints? Its function is only partially for stone protection.
Here's a for instance about heat shields. While rebuilding the motor in your old wagon, I found 4 pieces of rubber oil and coolant lines in the area around the up- and downpipes where heat shelds had been left off either the exhaust parts or the lines themselves. The hoses were cooked to death and came off in chunks. That motor had evidently been out of the car before, and whoever put it back in used a motor mount on the turbo side that did not have the WRX stock heat shield on it. Guess what was also cooked to chunks? Guess what kind of motor mount I put back in there?
Heat shields are not simply a PITA, they are there for a reason.
DG
BTW, PITA jobs are just part of the deal when you do your own work on a Subaru. this is why jacks, jackstands, and LIFTS were invented.