AFAIK, Koni have used the Vougtland springs for years. I even think it's what my original ones came with 10 years ago.

Anyway, generally, Mk4 springs will fit the ERST, but because the RST was lower than the Mk4 as standard, the quoted amount of lowering usually amounts to more than you actually get. So, a set of Mk4 springs quoted as 40mm lowering, will probably lower an RST by around 25mm. Of course you must take into account wear and sag of springs over the years, but thats roughly it.

I'd say you'll be fine.