Zoro is stronger using three-sword style because it's how he fights, and he has wider array of techniques to use. One-sword style gives him fewer techniques, so while he'd still be strong, he'd not be as strong. Has Zoro ever mentioned being uncomfortable in any way with fighting with less than three swords?
I mean, if Mihawk tried fighting two-sword style, he wouldn't do as well because he's not used to it and can't use his full strength, especially when he uses two hands to wield his black blade. Luffy's a great fighter, but if he got into a swordfight with Zoro, he'd lose simply because he has no idea how to use a sword. Same if a weapon is involved, or if Luffy uses martial arts.
About that specific point: Zoro has gotten progressively more comfortable at handling ichi and nitoryu, despite how utterly weak he was at those styles back when the series started (remember when the Nyaban brothers stole two of his swords and he suddenly couldn't even beat those two fodders).
The rest of the post just supports your point so feel free to skip it.
As he has grown stronger, both as a swordsman and just physically, he has also naturally become very strong with one and two swords now, and he has also been forced by the circumstances to win certain fights with only said amount (vs Daz Bones his other two swords went flying and vs Ryuuma he only had two swords on him) and he has proven that he's "strong" with any amount.
But, even then, as he has naturally grown stronger with ichiryu and nitoryu, he has become even more of a beast with three swords, his main style that he always polishes out the most, in which not only does he have stronger techniques, but also a whole lot more of versatility, and it's the style he always chooses to fight seriously for a reason (together with his bandana, but I can't explain that).
About the point on the Shishi Sonson technique that has previously brought up: It's true that the technique has more cutting power than anything Zoro had up to that point, but that was just an iai technique in which he put everything he had behind the cut. Very powerful finishing technique but not something over which to base a whole fightstyle. A good addition to his arsenal but very limiting when compared to his santouryu.
He did show later on a nitoryu iai, so I wonder if some day he'll come up with a santouryu iai (I hope not).