I don't think the Comet itself should be changed, however the Veterancy on the unit needs to be reworked.
Comet Veterancy
Veterancy 1 : Fires White Phosphorus instead of normal smoke
Veterancy 2 : +35% Weapon rotation speed , +30% accuracy
Veterancy 3 : +30% Speed, Crew will attack nearby targets with their grenades by default, +20% Acceleration
These bonuses are fairly useless compared to what other tanks get.
For people talking about the comet's vet, as far as I'm aware, lacking vet is a featureTM of UKF. Just look at the vet for infantry sections, cromwells, or sappers. Their vet is all pretty lacking. Obviously whether or not it should be a feature is a relevant discussion.
That said, I'd opt for a cost decrease. One of the other big problems of the comet is how much it costs in teching and the unit itself. You end up sitting on a lot of fuel for a long time. A lower cost should make it less of a risk to save up for/tech up to, and make its cost to performance ratio more generous.
Removal of the +30% accuracy and making it +10% accuracy as well as -10% scatter for vet 2, then giving it a -15% scatter bonus in addition to the WP at vet 1
MIGHT make it good enough to warrant its cost and power.
And to everyone else who wants this to be a "panther niche", essentially you'd be buffing the AT power of the comet a lot and probably require a cost increase, unless the AI is dropped to panther levels. Also that is what firefly's are for.