Your premise is to get the oil viscosity low enough to stay out of bypass. (while still maintaining the minimum oil pressure that the manufacturer considers low enough...on their recommended
viscosity to be worthy of a rebuild or remedial action).
What you are doing is dropping the viscosity (all things being equal, load, same engine) to increase the side leakage out of the bearings to get your magical OP result.
Chart below shows the side leakage to the total circulating oil volume in the bearing..ratio of 1 is all oil supplied runs out the clearance. ratio of 0 is all oil is contained within the bearing and recirculates...It's actually why operation on bypass is harmless to the engine (although wasteful of power, the higher viscosity leaks less (less backpressure in your parlance), but spends more time recirculating in the bearings...as I've stated before, self compensating to an agree)
Consider the L/D of about 1/4 as the discussion point, and 0.1 to 0.4 as being reasonable design conditions at design point.
To get the oil pump out of relief, you are running on a fairly flat part of the curve, which requires a fairly large change in Sommerfeld number (viscosity all things being equal) to effect a small change in leakage flow, the leakage flow that is required to close the bypass.
Consider that change in the MOFT chart below, where going from 0.4 to 0.1 is more than a 50% reduction in MOFT.
Chasing your oil minimum oil pressure, bypass closed target, particularly when min oil pressure is by definition on the SPECIFIED grade of oil) reduces the margin of safety in the bearings.
You've stated that it will always remain adequate, and bring in your belts and braces drivel, but you have no idea (other than lack of catastrophic failures) where you are on any of this.
MOFT further depends on load and RPM...your "spirited" driving is different to the average Astra driver, with considerably more weight, and much more moving from a standing start.