It would not surprise me to hear that the McLaren layup is significantly stiffer than the stock layup without being heavier (which is all that matters to the Pros).
if i remember correctly the mclaren venge came out heavier (stiffer, but still heavier) than the stock venge. it was a joke, just as this is a joke. ax-lightness vial evo is almost 200g lighter and just as stiff where it needs to be; im sure a pro would have no problem with an vial evo. So what does that say about the mclaren and specialized engineers? maybe they should hire ax-lightness to redesign/maximize the carbon structure in their hypercars and crazy overpriced bikes?. its a case of dont give the people too much too soon, so you can improve on what you already have, then they can repackage it and sell it as the next coming.
Take a ducati 1199 the cost is $19,000. they also have an S and R with better suspension and slightly upgraded engine internals for $24,000 and $32,000. now comes the 1199 superleggera for $65,000. basically an R with a few material changes. they doubled the price and made it limited to 500 bikes. everyone knows, that for the money, these bikes arent worth it. my friend bought into the desmosedici which was $75,000 - the bike sucked!!! it sounded great, but it sucked on the track.
people will always buy thses limited bikes/bicycles. they have no real value over their counterparts, but it appeals to some, who cant see past what they are - mostly marketing.
The price is obviously extreme and high, but Specialized has never tried to make the lightest bike, they try to make the best riding bike. They don't seem to care that the bike isn't competing with the lightest frame sets out there, and only few people, or maybe the pro's would really be able to tell or feel how much different a McLaren 12R carbon is going to ride and compare to an S-Works 11R. Of course a slight bit stiffer (10% or whatever) doesn't justify 10k more for a McLaren. As a consumer in general, it would have been nice if they had additional selling points or features, built at McLaren exclusively, much lighter frame weight, etc. What I would like to see is a white paper from Spesh similar to what Cervelo did with the RCA. As you mentioned, many of the McLaren Venge frame sets were actually heavier than reported and the same or heavier than the S-Works. Spesh really didn't have a good answer when everyone asked why? I really like how Cervelo mentioned in their white paper, how they bought different frames from all manufactures, and actually weighed them showing the min, max, and avg. The proof should be in the data, so it would be nice to see more of it.