then I'll just make concessions on the appearance.
I don't see any concessions there, the grooves don't get bright red edges.
These are truly miracle grooves that hold back wind-driven water!
Nonsense, no miracles. The windowsill is inclined lengthwise but level crosswise >>> Consequence: precipitation flows off evenly across the width. But the lab ideal of calm wind and accordingly straight falling rain is rarely met. The deflected driving rain is pushed aside on the windowsill according to the wind direction, so on one side towards the reveal, and would splash higher there. In this way, a groove is offered to it which carries away part of the water. And a not insignificant part at that, since the groove is several water layer heights deep. No witchcraft – just plain drip groove or whatever it's called.
And we are currently not talking about aluminum windowsills!
With those, a upstand additionally provides stability for the overhang.