So...how bad is the deadzone using a flightstick?
 

Just curious because in Ace Combat it is horrendous... it's not the fault of the stick though it's just the programming of the deadzone within the game.

It is there. It sucks.

It is nowhere near as bad as Ace Combat or HAWX.

I'm using the Ace Edge flightstick and noticed that the Spitfire has a bit of a dead zone, not bad, but noticeable. However, when compared to the Mustang I can tell that the P 51 is much tighter with only a small amount dead of zone.
With the Mustang when trying to draw a bead on a FW 190 I would generate PIOs until I turned down the elevator and aileron sensitivity by about a third and viola! ...almost no more PIOs.

Based on what I observed in the demo I'm going to venture a guess that based on performance levels, different airframes may have different dead zones profiles in an attempt to mirror real world handling characteristics.
Just a guess.

PIO? pilot induced ocilation? (spelling?)

I don't think there is really much of a deadzone. It seems to be more about how twitchy or smooth the plane handles like Steiner has observed. This is even more obvious if you switch from Arcade to Sim mode.

I would like to think that this apparent dead zone on the Spitfire is a result of the real life early Spitfires having more than average slack in their control wires, resulting in a certain amount of control fuzziness, but this is probably wishful thinking.

I've never been in the cockpit of a WW2 fighter but I doubt the mechanical controls were very tight after a few thousand hours of flight.