If you get an alloy one it will need to have swirl flaps also they are used to make the air tumble at lower rpm . Looking at the engine from front the hissing and soot would be concentrated on the left side of the inlet manifold . As with any repair sooner is always better than later it could fail on next journey or last forever . If your fitting a new part a replacement plastic inlet will be as good as the alloy one it's only after a 50k+ I think the alloy is better . With reduced pressure I would say a good egr clean would make a fair difference too when it's going to be removed to repair the manifold