reginos:
Porsche must have a huge profit margin on the GT4RS. I expect only a handful of these cars will be sold without the top of the range wheels and the Weissach Paket adding another 30.000 EUR to the already significant 140.000 EUR.
For sure, the RS moniker and the mid-engine configuration on the Cayman will attract some potential GT3 buyers even at this high price.
For the same or similar money the GT3 offers more, like the bespoke racing front suspension, the more sophisticated multi-link rear suspension instead of the Cayman McPhersons , R.W.Steering, advanced high-tech axle differential instead of the mechanical one in the Cayman, wider wheels and tyres. The weight of the two cars is practically the same.
Some people comment on the looks of the 911 and its dimensions. If you look at the figures the Cayman is not significantly smaller. IMO notwithstanding some not so successful styling details on the GT3, the 911 has nice flowing lines whereas the GT4RS looks to me a very busy design with too many inlets and appendages.
Nevertheless my compliments to the GT Division for having the confidence first to price a Cayman at close to 180.000 EUR and second to allow a Cayman performance-wise to come so close to the GT3.
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"Porsche....and Nothing else matters"
I am taking the Weissach pack but I am not takin the magnesium wheels. Too much hassle. Have two of those before, on the 918 and GT2RS. You have NO IDEA how careful I have to be with curbs and whatnots 
Plus, I like the edgier Aluminum wheel design better, the magnesium wheels pretty much is the same since the 918 days.
One problem with picking the 992 GT3, it's not the range topper and the better GT3RS is coming, while the GT4RS is the range topper for the 718 range.
But pricing are always like this, you full load a 4S whatever you are up there with a GTS, loading up a GTS and you are in turbo territory, adding options to a turbo means a turbo S make more sense 
718 range has it's own appeal, while it is the 'entry level' car for Porsche, it is mid engine and different feelings with the rear engine on the 911 range.
I have commented before, while the ultimate dimensions of a 992 isn't that much bigger than a 991 and for that matters the 718 which is on the same platform, the sightline on judging and placing the car is different and hence making the car seems bigger than it really is.
RS cars are always busier than non-RS, they aren't designed for looks, they are designed with function in mind first, the GT4 already have cleaner lines than the GT4RS, just like how a GT3 has cleaner lines than a GT3RS.
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