No need for there to be a standing start, in any situation the petrol will be quicker, it has better power to weight. It may need to be more aggressive on the gear changes than the diesel but that is not an issue.
I don't actually get the point about selecting higher gears than needed so can't answer that.
Lets say both cars are cruising at 40mph, both in top gear. If they both floored the pedal, the diesel might get to 80mph faster than the petrol, but even when including the time it takes to drop down lots of gears, then use them all again to get back to 80mph, the petrol would still win. The diesel is allowed to drop down gears if it wishes, but the outcome would still be the same, both cars trying their hardest the petrol wins.
People who like diesel usually like them as they are less effort to drive, maybe less effort to drive quickly. But ultimately they are still slower.
In application to GT5 like I mentioned earlier, I did a test with 120i and 120d same power and weight, with the 120d having a load more torque. And the lap times were almost identical after about 10 laps each with less than a tenth between them for the fastest time (the petrol was fastest,but no real winner). So even with same weight and power, and a huge amount more torque the diesel can not significantly win (but is about the same). The lack of power band balances the high torque.
But in reference to two cars where the petrol is actually more powerful and even lighter, it's a no brainer.