If this was addressed somewhere in the dozens of pages of Wave 9 discussion, please disregard.
The Tail Gunner card in the upcoming ARC-170 only states "Limited". It doesn't say "ARC-170 only" or "small ship only" or any other limitation. I would guess it's a Crew upgrade.
My flat out guess would be that this card enables ships with an aux arc to make an improved or a second attack out the aux arc. Or perhaps to avoid overlap with the titles for the ARC-170 and TIE/sf, maybe it enables secondary weapon attacks out the aux arc.
I'd be pretty happy to see the Firespray get some love.