Oil is oil, there is no oil that works perfect for every temperature range.
Compromises exist in a 5/40 oil when in super high temperatures.
15/40 oil will perform better simply because it requires less additive to make it more robust at high temperatures.
Its really that simple.
However, if you drive in normally cold climates, a 5/40 will perform better simply because it is more fluid, however, because of that, more additives are added to the oil to make it "thicken up" at higher temperatures and this is a compromise to make it perform like a 40 but its not a 40.
Bottom line, the less "spread" between the numbers, the better, everything is a compromise.
Will it matter? most likely not but, again, then why are we in this forum, because it can matter.
If your in a northern climate, sure, use a 5/40.
Southern climate? Use anything, you have the option of a more dedicated (less spread) specific oil for your above freezing operating temperatures.
Lower viscosity oils are simply because of the USA EPA where gas mileage (immeasurable to the vehicle owner) is more important then a POSSIBLE slight loss in engine life.