The Cromwell is 110 fuel, so it's cheaper than the Ostheer P4. It also has significantly lower tech costs, particularly in teamgames where it's doable to skip bolster.
UKF without side techs: 145 fuel for tech + 110 fuel for Cromwell = 255 fuel
Ostheer without T2 building: 155 fuel for tech + 120 fuel for PZIV = 275 fuel
But this is not realistic at all. Since UKF normally will build bolster and an AEC (and maybe grenades or weapon rackets) while Ostheer will build T2 and a 222, the PZIV will show up before the Cromwell does. You will have to fight the PZIV with AEC + 6pdr. That is a lot more realistic.
UKF: 145 + 35 + 20 + 60 (+ 10 / + 15) + 110 = 370 fuel (380/385)
Ostheer: 155 + 20 + 30 + 120 = 325 fuel
The Firefly does not have low DPM at all, it's similar to the Jackson. ROF is only an issue against certain targets like 640 mediums, where its 200 damage leads to overkill damage, artificially lowering its TTK. The TTK against a Panther is actually similar to the Jackson and only slightly worse than the SU-85. Vetted, it has the highest DPM of all TDs at 2304. Its vet 3 240 damage makes it significantly better against all targets (mediums from 4 to 3 shots, Panthers from 5 to 4, etc.).
This is excluding the Tullips and the doctrinal Command Vehicle bonus.
Technically Jackson has higher DPS, here for Vet0 to Vet1 at least:
Jackson: 160:4,675 = 34,22
Firefly: 200:8 = 25
At vet2 both gets a reload bonus, this will lower the technical difference in DPS, because the FF profits more. At et3 FF gets 40 more damage, Jackson gets a further reload speed. Technically Jackson still has some more DPS at Vet3, although FF always has higher alpha damage from Vet0 to Vet3.
Since - as you said - FF generates overkill versus some targets its real DPS is even lower. I'm not calculating with tulips for FF and AP-rounds for Jackson, although AP-rounds generate somehow guaranted damage with a great Penetration boost while tulips often do not connect (sometimes just because of terrain).
But the main thing is that battle situations are not static. If you get assaulted by tanks like Descolata wrote about for example, Jacksons will handle that situation significantly better. Same if you want to pursue a crippled tank. Speed, Accleration and moving acc have a way bigger influence on real DPS than the slightly better acc of FF when not moving or its higher mid range.