Power talk incomming!
Cheapness of Luffys gear second is why I thought about limitation it has to have beside hacking away life and I expect gear second to be boost to Luffy but stronger Luffy becomes less of a boost it gives to him.
I would say that Lucci could have been between Rear Admiral and Vice Admiral while Luffy was A LOT weaker without gear second- he didnt instantly KOed guy with 800rokushiki? I know same like Sanji didnt broke at first through tekkai os same person but still. Gear Second gave Luffy amazingly big boost before timeskip because it pushed his body further far beyond its limitations. Now those limitations are not as low as they were.
Its like with real dopping - it pushes your body but if you train without it you are set to get to same limit as everybody else, you wont break all records by far because body capability also have its own limits.
Now Luffy should be fairly stronger, his body on its own should have also gotten stronger so I expect that gear second still gives him a boost but it still should be avoided as much as possible and it doesnt make him SO MUCH stronger than it made him before timeskip.
But I sure do think that Luffy will be able to pummel Doflamingo with CoA hardening on whole body + gear second.
Also you have to add that Luffy now is kind of pushed into using gear second as he should be fairly tired by now after all of fighting he did till point where he is and after getting attacked by Bellamy and probably taking hits for Bellamy not to die.
Also Gear Second and Pills that Hody and gang took have same price as they hack away life(gear second doesnt make your body change though it costs life force to use new power). In different ways but still it might be why Luffy avoids using it as much as he can and thats probably why he together with Rayleigh came up with partial usage of gear second on Luffys body.
Btw I liked how Dellingers attack that finished off Ideo made me remember about how similar attack landed on Luffy here
!
I think that Dellinger is weaker than Hody - at least less durable and physically weaker - because I dont expect Delli not to be taken out by CoA hardened and jet punches.
Luffy tired after fight with Chinjao was still able to fight back and stop Cabbage(Luffy had no interest in fighting him) and even with boost that Hakuba gives to Cabbage I expect Luffy to be simply superior to Hakuba, so If Hakuba can one shot Dellinger then Luffy should be able to do the same.
It also adds to how strong Luffy has become if he can "tank" attack as strong as Hodys and still continue to fight well untill using Elephant Gattling that torn wounds made by Hody.