I'm sure they did although what was 'acceptably light' back in the 80's wouldn't pass muster these days. But yes, the undertray should help.
Be interesting to see a wind tunnel comparison done with and without. Also rubber rear spoiler vs hoop vs 968 turbo. I doubt even the factory had a wind tunnel back then? Although we all know that silhouette drawing with the pressure map of 924. Assuming that was done by the factory.