Hi

Ive a 2008 impreza (1.5r on a 08 plate)

rear heatshield under the rear silencer is vibrating like crazy, had a wee look and the fixings and washers are still in place but the thin metal heat shield has broken free, it is closest to the N/S and is approximately 2ft x 1ft

Options seem to be to either try to get the rusted fixings off and replace with 50mm washers or just remove it completely.

what would be the risk of just removing the heatshield?

 

I'm having a similar problem with mine but I haven't had a proper look at it yet. This video looks like it might have a good solution:

 

Got to be worth a bash, only talking about three jubilee clips. I think I’ll try and get some stainless steel ones on Amazon

 

Or stainless cable ties used to hold exhaust tape on.

 

The video is not much help with your problem.
The larger repair washers sound like a good idea.
Part number is "cover complete exhaust - rear" 59024FG020



and yes it helps prevent heat transfer into the hatch area.