that was 2 years ago. luffy has no drawback for g2 and g3. he might not even have drawback for g4 anymore.
As far as I have understood luffy powers.
On why he is stronger than other supernova is Luffy uses this Haki blast and full potential of Devil fruit right of the bat.
Luffy gets More power for less duration.
Kidd and law uses Less for more duration.
Kidd and law were getting ready to fight Kaido once luffy was down.
When luffy figured out Adcoc I'm pretty sure luffy couldn't keep up with a prolonged fight. Same stuff happened in next fight. Luffy himself said this is his last g4.
I'm not trying to downplay luffy here. But luffy becoming a brawler like garp at base form with haki will do wonders and using G4/G5 only as final resort to increase his Endurance.