When I worked at a VW main dealer we sometimes used to phone GTI ENGINEERINGÂÂÂ

for advice back in the 80s as we used to find them a lot more helpful and knowledgeable than the mostly useless buggers at VW SERVICE TECHNICAL DeptÂÂÂ

They were quite popular in their time , the conversions that springs to mind was the RE 2000 Golf mk2 GTI and a White and silver Jetta GTI with BBS body kit on it ( I think ) ,both pretty quick in their day and probably still areÂÂÂ

( if you can still find oneÂÂÂ

)
I think they were based either on or very near to Silverstone