OK Young man it looks like you're majoring in litterature.
People usually don't get the model from their pocket...even Vayer. Vayer uses Portoleau's equation and Portoleau, like Grappe, Ferrari and many others have developed his equation /model to calculate the wattage developed by a rider during a climb.
You input some parameters : weight of rider, weight of bike,
length and gradient of the ascent (at least) and for the more complex models you input frictional forces, wind, Cx etc... Press the ENTER key and you have estimate the watts developed
Porotleau's model is good because he can oftens validate the results of his calculations with SRM data provided by pro riders.
Ferrari's model though quite simple is quite good... no surprise

So when Portoleau's model say its 6,7 Watt/kg it may be 6,6 or 6,8 but no more than 2% error margin.
The only problem for all the models is that Froome's weight is a secret data. Like for his VO2 max he or team SKY never really answered that question.
Why? Maybe because that way nobody is able to calculate Froomey's power output