Grooves, in my experience are good for "casting off" road spray - more so for cars where the callipers are at the rear of the hubs and so subjected to more incoming road spray, particularly in winter when it has a high salt content in some areas of the country.
ATE Power Disc worked well on my 2000 VW Passat 4Motion where initially there was no protection/prevention of shielding the discs and pads from getting covered in road spray, same age Audi A4 eventually got additional plastic parts to prevent a lot of that, and the next or facelifted B5 VW Passat also got that along with proper sized front discs.
Where things went bad or wrong for me was that I started to use my car less in winter, and that meant it ended up getting garaged at times when salt rich slush/snow/ice had accumulated on these grooved discs - and that quickly lead to serious rusting progressing out from the edge of the grooves, and soon it got that bad that it did not get cleaned back to bright metal after a few miles of use - that lead one pair of ATE Power Discs to be scrapped/replaced before the discs wore down to their useful limit, that was annoying!
Neither of our current cars seem to suffer from that horrible "washing out" of the front brakes in extreme winter conditions which lead to zero front braking effort for the first maybe 10 seconds of braking, so I've not needed to repeat swopping plain discs for grooved ones.
