They wont fail you for having it de-catted or having a modified exhaust that is non standard.
They will fail you on emissions or if there are any leaks/defects on exhaust system.
not sure about the impreza exhaust system but i have 2 cat sections in the legacy, if the one closest to turbo is removed, will car still past emissions with just the one smaller cat still in place?
Are your`e emissions test different than n.ireland as a fully de-catted car will not fail mot.
Its tested on idle so car would need to be running very rich to fail