Very interesting argument! I still think that Luffy's ability to keep standing is what makes him strong, and yes, being a rubber-paramecia helps him with that...but if he was at his current power level (Dressrosa) and instead of 'Gum-Gum Hawk Rifle' it was 'Entire Gigantic Volcano Hawk Rifle' or 'Completely Freeze Someone's Blood In Their Veins Gatling' he'd, in most general cases, be stronger. But there's more to it than non-specific 'power levels' vs. mooks he might hypothetically fight. I mean, in terms of crowd control/dueling I still feel Mihawk/Shanks (normals) could take almost all the DF users we've seen, at least 1v1, and have huge AoE with few drawbacks.
The thing about this series is that luck is a superpower. When Luffy survives execution, all Smoker's reaction establishes that it's literally a sign from the heavens that he has a grand destiny. (I know Dragon was there to get it done, but what were the odds that they'd both be there at the same time and willing to expose himself like that, etc.?) Repeatedly (against Enel and Lucci most obviously) his specific, weird powers (rubber, hard to punch) are exactly what are needed for the challenges in his way. Not to take anything away from him, he works hard and is a beast in battle and I love him, but his biggest similarly to Gol D. Roger is that both have a child's nature and the devil's own luck. I mean, Haki is basically just "The Concentrated Power of Being Really Determined and Awesome", so this series is all about metaphysical personality stuff having real power.
So his Paramecia? Exactly what he needed in the future, ideally suited to his psychology ("Punch bad people! Then keep punching!") so the universe put it in his way. Just like it set him on a series of trials that give him new abilities in a gradated fashion, provided him an elite Haki teacher, introduced him to genius allies...all the usual coincidences of Shonen manga are, I believe, specifically stated in this series to be Luffy's in-story power that 'fate is on his side'. Even when he is outclassed (Ace's execution) he manages by unbelievable luck to get away in a manner that ends up with him stronger.
So Paramecia vs. Logia, and even the level of one's Kung Fu, aren't the big deal - it's Haki, which is basically Luck + Will + Batman, which Luffy has in spades because the gods want him to inherit Roger's throne.