Would this be a direct compatibility? Any negatives?

An A3 has a base weight of 3263 Lbs and a GTI has a base weight of 3100 Lbs - a 5% delta. Or +/- 2.5%. That's within the manufacturing spring rate tolerances, and certainly within a few degrees of shock adjustment tweaking.

For the record, a GTI is also 2.4" taller and 3" shorter, on the same wheelbase, for those interested in how things like rollcenter and polar moment of inertia impact handling.