The fact that the Firespray was a pre-Clone Wars design,
Only the Slave I is an original Firespray, and that's a ship that's been modified countless times by both owners, undergone multiple overhauls and a complete refit by KSE.
The other Firesprays were manufactured by KSE in the Imperial era.
See my above post, as I think it answers this quite accurately. What we're talking about is not a new ship called the Firespray, but the original Firespray that you are assuming (on the basis of zero actual evidence) to have upgraded components. Maybe it does. But if you can update the ship without changing its appearance, and we have N-1s making a flyover of Theed in RotJ, then the same argument applies to the N-1. Given the wonderful characteristics of the airframe, that is suggestive of the original argument I was making, which is that an N-1 has 3 agility dice. (Though, I confess, it has been a long and torturous path in support of those 3 agility dice).
Edited by Nightshrike