@DHL:
Well really, for all we know, weakening someone could also weaken their Haki resistance, reducing more damage than it could. That's speculation.
Doflamingo was saying Luffy had the speed after he was weakened as well, before that Luffy couldn't do anything and had already been outclassed beforehand.
You probably don't see the difference because Doflamingo was still spanking Luffy. You're judging gear 4 based off the chapter with the title "gear 4" which was created to showcase gear 4. Of course it's not going to show the gears drawbacks, and show Doflamingo easily countering it, it would give no one any hope that Luffy cannot defeat Doflamingo and make Gear 4 look shit.
Doflamingo who is at the brink of death, only living because his organs are being patched up was still beating up Luffy who was earnestly trying hard.
Doflamingo has been coasting this entire arc. He has been toying with Luffy thus far using his brute force. Now he's going to showcase his own power. Trust me, even though Luffy has the power in his attacks now, Doflamingo will exploit his weaknesses quickly. Even though he'll lose, he's not getting stomped.
To be fair as well, we've never seen a full strength all out Doflamingo, we will only see his supposedly weakened self.
A lot of people were confident that Luffy would defeat Doflamingo with his current gears with high diff. No, Oda thinks Luffy requires Gear 4 to still take on Doflamingo all the surprise assaults.
All out Gear 4 Luffy = Gamma Knifed/Countershocked Doffy.
The sad thing is I don't even think they're going to delve into the drawbacks before Doffy is defeated. The mode (speculation here) "bounce man" could just be his starting point there could be more to come, so there's more of a beating on it's way. Gear fourth luffy doesn't equal counter shocked/ gamma knifed Doffy yet because as you put it the showcasing chapter showed Luffy beating Doffy by a landslide, there is no equality in that. In subsequent chapters maybe he'll utilize his string abilities more to even the playing field instead of relying on brute force so I do hope to see that and I hope you're right because a one sided fight can never be referred to as a fight, it's called a massacre.
–- Update From New Post Merge ---
@DHL:
Body swap trick twice? Doflamingo didn't expect a supposedly dead Law to exchange with Luffy.
That, my friend, is exactly what I meant by undermining Law's intelligence. The lack of anticipation is baffling considering Law has survived so much beating already he had to resort to splitting his skull with a stomp just to ensure he doesn't come back to haunt him.
I can't believe you had to add "if he had" who in the arena could beat Luffy? diamanté? same diamanté that got offed by a nameless slash? Burgess? Lol