My thoughts regarding the cage ... Adds more weight yet greater rigidity ..?! Adding more weight would have therefore meant the intervention of CCBrakes to mitigate weight gain, therefore adding huge cost to retail price point?! This is a Golf GT3 not a Porsche GT3 at over £130k - everything has to be done with overall pricing taken into consideration and at £34k that is approx 25% of the cost of a GT3 ... Secondly, does the car need a cage "No" the idea of the CS-S and the Golf GTi culture is useable daily performance and adding that level of rigidity would then need revised suspension, tyres and the whole beauty and design brief of the CS-S is the soft damping for Nurburg and English B road Tarmac, too much rigidity wold see the car bounce round corners, add to that FWD and you have a nightmare car on the edge round a bumpy corner, exactly what we have here in the UK .... VW have thought long and hard about this car and what values it needs to have and what goals and expectations it needs to cater for, all with a limited 400 build and a £34k price tag .... "Brilliant" 
The car is great first and foremost but I would disagree a little
Adding a 12kg rear cage wouldn't mean CCB were required, Why would you think that? the heavier R (100kg) doens't need CCB's so why would a 12kg cage require the car to need them? The cage would add atmosphere or theatre to the car which as limited Edition is pretty mandatory. We know rear cages aren't required or needed but the Trophy R or R26R looks all the better for them imo. A cage won't make it too rigid either, unless you weld it in at multiple points and then it wouldn't be a half cage (which is all that is required). Adding to the already rigid shell (compared to other hatches) wouldn't require revisions to tyres or suspension.
The CS practicality and "usable golf" is gone or reduced the moment you take a golf and remove the rear seats, so why not go all the way and cage it? It did the lap time with the cage, so why not? Its not like people are sitting in the back area? The bar fitted with the net is too high up to tie the rear mounts together so it serves little structural purpose i guess.
I think they could have made a couple of small changes: For 2k like Brembo's, the Ring time half cage in red perhaps? This would add to the look, which after all is what this limited run hot hatch is all about imo and also allow users to fit harnesses.
But like always the bean counters get involved...