I have parabolics on my 88" S3 and i am very happy with them, fitted them 5-6 years back. They are not rocky mountain brand, i got them from the UK.
Mercedes Sprinter vans have single leaf parabolics on the rear and they ride well.
Just one thing on the standard springs to get them moving freely is to jack your vehicle up by the chassis and let the axles hang. Then spray liberally with innox and let them sit for a day or so. I used to do this to my original springs every 2-3 months and it realy frees them up and gets them moving. The only downfall was it made the springs clack a bit as the leaves moved over the worn sections on the adjoining leaf. Nothing a brush with an angle drinder wouldn't fix though if you stripped them down.

Cheers, Mick.