They only ran it very briefly I believe, I can't recall what the site I got it from anymore though. The weight effect is significant if you think about it, 2 extra camshaft, bearings, valvetrain...etc, and most of these parts are likely to be steel instead of other material, and all on top of the engine.....

Back in those days flat engine like Porsche's are also difficult to package as they take up too much room in the back, room for the all important underbody tunnels on group C car. So much so that even Porsche needed to tilt the engine in the longitudinal direction by 7degrees....