This sounds like two different things reported here, and only one is a problem.
With default controls, pitch+yaw, a diagonal input seems to move the point of aim the same rate as on either axis alone, and that is totally desirable in my opinion because you move the stick in a circle and the point of aim moves in a perfect circle. This is needed for muscle memory. In the real world if a stick controlling pitch and yaw worked differently, I'd call that a bug because diagonal point of aim should never speed up.
The problem is with the AVIATOR controls, pitch+roll. When you give full pitch input, and slowly rotate towards the side, the problem is obvious. First your pitch rate drops gradually to about half - with zero roll up to 45 degrees. When you hit 45 degrees it feels like half roll suddenly kicks in, with your half pitch. As you rotate towards the side pitch continues to bleed off to zero as roll rate increases to the maximum. Roll shouldn't be ignored in a huge cone on the y axis, and pitch shouldn't bleed off when the stick is held slightly off axis when yaw is not the other axis.
Full diagonal input should probably be mapped to full pitch and full roll. Like a square mapping. Stick mashed up from -45 to 45 degrees should be full pitch at least. From 45 to 0, pitch should slide from full rate to none. Maybe slightly different mapping would feel better with circular joystick movement, but full pitch should always be available in a large arc, and roll needs to be available when the stick is left or right of center in any condition.
