>>283216056
>Is it ever stated that using hatsu messes with how much aura your Ken receives?
Yes. Knuckles notes that when Gon activates his hatsu his Actual Aura Power (A.A.P) in his fist went up about 200 from his base 1800. It can more easily be seen with other enhancers like Phinks where their aura output drastically shoots up for an attack.
>would his Ryu get weaker while he's doing that?
Ryu is the time it takes to move aura proportions around. It shouldn't get slower just because the user is making an attack but it would require good training to keep up with things. Naturally the less aura you have distributed to a location the weaker that spot is.