Comet already serves to synergies with the firefly. The firefly is tuned for heavy armour and high health pools but suffers at the hands of mediums due to its speed. The comet kicks medium ass. It's last round of nerfs was to make it so it WASN'T a panther analog and instead its own thing. Revisited vet would be a good place to start. It's not a panther it's a t34/85
OP has the right idea though.
The problem with the Comet is that not only it was nerfed, but that the nerf was multi-faceted
In it's original form, Comet was powerful on its own, and borderline broken when massed, so? it had both its performance nerfed, and popcap increased, so now you end up with a vehicle that's isn't impactful enough to justify it's cost if you only get 1 of them, nor is it viable to mass it due to it's price and high popcap (unlike, say P4, or Panther in team games). Anyway, it's not all that useful, because Churchill can bully mediums as well, and it has much better synergy with firefly anyway due to its health.
Comet is a tank with high cost, high popcap, 50f side-tech, cromwell level AI (so, worse than average) and ok-ish AT, but not much more than churchill (as in, still can't realiably fight panthers, and deals with mediums slightly better). Its in a really weird spot of being "the most premium of mediums" and it suffers for it.
Less than mediocre vet doesn't help either, that definitely needs to be looked at, regardless of any other changes.
I'd say the formula for Comet needs to change
Either we make it closer to other "generalist-premiums" by lowering it's armor to 210/220, range to 40, pen to 160-140, buffing its AI to a decent level, and lowering the price and popcap accordingly to make it possible to mass (34/85, Sherman E8 sort of thing)
Or we turn it into Panther-style AT-brawler: decent health, armor, 50 range, more pen etc. then we nerf it's AI and call it a day. Tank destroyer than relies on diving and getting closer rather than sniping from 60 range
OR, we buff it, turn it into a mini-pershing (or Tiger, whatever), buff it's AI, 50 range, more pen and whatnot, increase the price to 200f and limit to 1 on the field, so you can't spam them.
Make it so its impactful enough when it's alone, or viable to have multiples of, because right now its neither.