I stand by my original wording. At extreme temperatures (ABOVE normal operating), the 0W-30 provides BETTER protection. If you graphed viscosity vs. temperature on a log scale, you'd create two straight lines. Both lines would pass through the same viscosity at operating temperature but the cold start viscosity is lower for the 0W-30, therefore creating a flatter sloped line. As a result, your viscosity at extreme temperatures is HIGHER with 0W-30 than it is with 5W-30.
I think you're a bit off.
A better indicator of what you're looking for is viscosity index. The higher the VI, the less the viscosity changes with temp (which would be a shallower ramp, not a steeper one). That means it thickens less at low temps, thins out less at high temps. Typically a 0w-30 will have a higher VI than a 5w-30.
1. if the 5w-30 is made with much thicker base stocks than the 0w-30, it might still be thicker at all the hot temps it'll actually see
2. there's more than one way to get a high VI, and some methods leave the oil with less shear stability. so, in the hottest parts of the engine, the shear forces might make the 0w-30 thinner than its VI would suggest at that temp
so, if you're comparing Mobil 1 5w-30 to Castrol Edge 0w-30 SPT, the 0w-30 will likely be thicker when really hot because it'll hold its viscosity better
but if you're comparing BMW 5w-30 to Mobil 1 AFE 0w-30, the 5w-30 will probably be thicker when hot. One way to figure this out would be to look at VI and HTHS viscosity.
if both are high, that's one hell of an oil when hot
high VI + low HTHS = really not shear stable
low VI + high HTHS = likely suitable, though might be thicker than preferable when cold
low VI + low HTHS = terrible
(this is all assuming you're comparing oils within a given viscosity grade, e.g. xw-30)