rather than 'inventing' new cards and rules (which I did do), there could be a way to produce Stealth X within the current rules with a slight adaptation.
The R2-F2 card provides an action to increase Agility and the Stealth card allows it again until you are hit.
If you were to allow multiple R2-F2's then Stealth X's are possible and quite realistic. They are also balanced.
I say balanced as you have to pay 6 points above a normal X-Wing to get one, you must sacrifice actions to get the full benefits and when you are hit you become more 'visible' just like the Stealth X's did.
Also, once in combat, you're likely to want more actions like target lock, once again reducing your agility and making it, realistic.
In all, you have to make tactical decisons between being hard to hit and hitting back.
You can make one pilot Stealth X as is. So the only house ruling is more than one R2-F2 being allowed.
Would you allow it?
Keep Calm. Carry On