Sounds like you have a "GTI Engineering" car. Car supplied by VW to, and converted by GTIE, were seen as a VW product, and retained the full VW warranty, in the same way that Ford and Cosworth worked together as one.
Gets some pics up on here, as its possible that it may be a known car

Either way, GTIE or not, it sounds interesting.
Edit, VW Heritage in Germany offer a service where if you supply them with the cars VIN number (anyone got the link) they can give you a certificate that tells you exactly what specification requests were made, this should include and mods done by the likes GTIE if the car was ordered through them from new.
Thom