Firsly, listen to whatever P-dude says, sounds like he builds more engines in a month than I'll build in my life.
For those that find it interesting, some more info on the subject:
Grooving a bearing does affect the load-bearing capability of the bearing when the load bearing portion of the bearing is grooved.
As P-dude pointed out, the crank is all over the place during operation, so the direction of the crank load on the bearing changes quite a bit and changes rapidly.
The groove is a 'necessary evil' in some applications to insure oil feed into the load-bearing oil film is sufficient. If the load on the crank was always unidirectional - it did not change all the time, then grooving would not be necessary, but with the extreme amount of activity you get in the crank with a high-powered engine spinning very fast, and the direction of the load fluctuating wildly, it is better to sacrifice some bearing surface and insure adequate oil supply.
On an engine that does not need grooving to insure good oil supply to the load-bearing film, grooving on the lower half will at best do nothing and may actually promote wear.
Since the load most of the time (on a more 'mellow' engine) will be against the lower bearing half, I suspect that's why the upper half is grooved and the lower half is not on some factory bearings. The groove on the upper half does no harm if the load remains confined to the lower half of the bearing.
I do not have firsthand experience measuring the difference, but would suspect that the grooved bearings have little effect on oil pressure; the main effect is oil viscosity and bearing clearance. Grooving may lower the oil pressure by a barely noticeable amount in theory.
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