So the grooving of the center main bearing is not to provide better main bearing lubrication but better oiling of rod bearings. Main bearings are generally not being taxed as hard (same bearings used in 400HP GT3). But getting oil to rods can be a problem. Picture below is from a old article showing oiling of stock 911, 908 and 917 crankshaft. The 911 at 9K rpm has lower pressure on center rods. For the 908 they had to raise the oil pressure to 100PSI to get the center rods oiled. The 917 had center lube and had better rod pressure at 10K with only 34psi. So drilling of main journal is to get more oil to the rod bearings furthest from the end when using engine at high RPM.
john