In the book B-29 Hunters of the JAAF by Henry Sakaida, on page 26, he says the Zero and 43's maneuverability meant that they could maintain a high altitude better than the 44, even if the 44 could climb faster.
We need to break this down a bit.
So there's two ways that the A6M and Ki-43 could "maintain altitude". First is that as energy is expended from maneuvering, they do not develop as high a sink rate due to the lower wing loading (and thus a lower Vmin). However pilots should not be maneuvering much when attacking bombers.
Climb above, get ahead, dive in from the front, sides, and directly above. Pull up around into a zoom climb, reset, rinse, and repeat.
Second is that they won't require as gentle of a pullout from the dive.
This is potentially relevant.
However (and I readily admit that this is only of limited value, due to being simulators), having flown flight sims for over a decade, every time we had historical scenarios run, the largest issues faced by the Japanese team were simply reaching the bombers in the first place.
We'd be patrolling at 30,000, and have the bomber streams come in a few degrees off their expected vector, which puts them 30+ miles away, and even once we ditched external fuel and floored it, that still puts them on final before we reached intercept. And ordnance free before we could knock down a significant number of them.
Knocking them down quick meant ignoring the guns, but then my fighter always got chewed to shreds. You'd be lucky to get three, maybe four passes before something absolutely critical to controlled flight gets buggered. Or maybe you just catch a .50 with your teeth. Or get set on fire.
And any escorting fighters virtually guaranteed failure. In 8 different scenarios, I only managed to kill two bombers during a contested intercept flying as an IJAAF or IJNAF pilot. And that was even flying Ki-84's a few times during some of the later scenarios.
The damn planes were just never fast enough to do what I needed them to do.
And all of this is supported by historical evidence as well, from combat reports, to the trend of increasing speed, even among the reluctant Japanese.
The entire history of aerial warfare underlines the supremacy of speed.