That's the whole point. Luffy's stupid when it comes to common sense, but he's a tactician in combat that's unrivaled save for a few. I'm trying to point out that what Luffy knows, he knows well. What he doesn't know, he doesn't have a clue about.
All the times Luffy has showed intelligence has been either combat or physics. You're right, firing the cannon into Laboon in itself was stupid, but the physics application that Luffy understood wasn't. Just like Gomu Gomu no Baka. It was an idiotic move, but Luffy understood that if he wasn't thinking conciously, Enel couldn't predict his moves. Same with when he was hitting against Maxim to pummel Enel.
Okay, I'll concede. Maybe blood doping is a bit beyond his scope of expertise, but look at this way: Compared to the original Shonen Superhero, Son Goku (Well, the ORIGINAL ORIGINAL was Arale Norimaki from Dr. Slump, but that was a gag series, really), Luffy's a genius. Goku would come at an enemy until he lost, then go train for a bazillion days, then come back and beat him. Luffy will actually modify his combat strategy. When he was fighting Krieg, for example, he realized that he couldn't finish breaking the armor till he got rid of the spear. So he started hitting the spear to break it and get it out of the way. With Crocodile, he realized blood works the same as water, and blood can't be evaporated like water can because of the plasma within it.
Is Luffy stupid? Yes. I won't argue that. He's not very bright. But even combat experience can't teach you everything on the spot. Luffy fought with only Ace for 10 years. No one else in Fuschia village was even close to his level. I doubt Ace varied his combat ability enough to account for ALL the different fighting styles Luffy would encounter. I'm sure Ace couldn't create Air doors, didn't have armor with 50 weapons inside, didn't have claws and could move incredibly fast, etc, etc. Ace was a normal human and Luffy's only known training partner.
That said, Luffy analyzed his opponent's abilities and adjusted/created moves on the spot. That's the mark of a very observant individual and a leader.