Depends on vet and hp pool but generally FF kills higher hp pool tank faster
No because FF bounces a lot on Axis heavies meanwhile SU 85 gets extra pen with vet.
SU 85 vet 0 far pen: 220
SU 85 vet 3 far pen: 286
Firefly vet 0 far pen: 210
Firefly vet 3 far pen: 210
If we take the example of KT
SU 85 vet 0 and 3 chance to hit at max range: 100%
SU 85 vet 0 and 3 chance to penetrate: 58,6 and 76,2%
So if we factor in chance to bounce on average a shot from the SU 85 will do 93,76 damage and at vet 3 121,92.
The KT has 1280 HP so it will on average take
13 reloads (reload+ ready and fire aim time) at 5.65 or
11 reloads (reload+ ready and fire aim time) at 4.57 (vet3) to kill it.
Which makes TTK.
Vet0 SU 85: 73,45 seconds
Vet3 SU 85: 50.27 seconds
Now the same with FF
Chance to hit at max range 100%
Chance to pen at vet 0 and 3: 56%
Average damage per shot vet 0: 112
Average damage per shot vet 3: 134.4
So at vet 0 it takes 11 reloads (reload+ ready and fire aim time) and on vet 3 9 reloads to kill the KT on average.
TTK:
90.65 seconds at vet 0
56.25 seconds at vet 3