I think some people think their GTI/GTD is a Ferrari. A top end Transit Tourneo has just as complex a build as a Golf, the end result might not be quite a pleasure to drive, but no less complex - especially when you factor into the equation custom fleet builds that differ from main spec because BT, Dynorod, British Gas etc require a non standard interior like tooling racks etc.
Their list of optional extras are no less complex than VWs offerings - eco tech, quick clear windscreens (as standard on the bottom spec!), rain/light sensors, complex entertainment satnav...the list goes on. You can even specify your tyres, such as Goodyear mud and snow, Goodyear allweather, Continental allweather, 3 brands of summer tyres etc.
How much manual intervention do you think a GTI gets over a standard Golf? It's not a hand-built Bentley, but all Golfs will have some manual fitment, there are some tasks that robots will struggle to do from a limitation of access - most of the internals will be manually fitted with air tools on all Golfs. Almost everything you can fit (or that is standard fit) to a GTI/GTD is speccable on a lesser Golf, from a tooling point of view there is nothing uniquely required for the GTI/GTD. Different grilles and trim are screwed on the same way as lesser models, using the same fixing points, as are the "unique" seats, steering wheels and gearsticks.
I used that as an example as I used to work at the Ford Transit plant and saw all of the processes in detail, modifying lines to accomodate more efficient layouts for the workers, new robots etc. All car plants are complex because their product is complex - just down the road from me is Sunderland Nissan, the most efficient car plant in the world, a centre of excellence in the automotive industry.
Are GTI/GTD really built on a different line to the other models? Haven't seen anything to indicate this, and from a production point of view it would be massively inefficient for such a low volume model variant which shares probably 95% of it's components with a GT to have it's own line.