If you compare the best apples-to-apples images from the excellent 2016 Comparison thread, you can see rear diffuser heights relative to the driveshaft/rear-lower control arms. The pictures are low speed, so near static ride-heights? It appears the "rake-leading" Red Bull has ~ 10 mm more diffuser height than the Williams. This difference occurs over ~ 3000 mm wheelbase, so rake angle difference (if leading edges of tea-trays are same height) works out to 0.2 degrees. This seems irrelevant, even by F1 standards. Maybe the significant of additional rake is how it changes the vertical position change of things like front wing, rear wing, diffuser, etc. But here's the part where I admit I have no idea.