Regarding the cons of the command panther: "Veterancy bonuses only improve the aura. The Command Panther does not improve."
The unit profile (and I think the description too) says
"Rank 1: Adds the ‘Combat Blitz’ ability, increases the movement speed of the Panther at the cost of rate of fire and accuracy.
Rank 2: Veteran tanks receive armored skirts as well as improved weapon tracking. Command aura now increases weapon accuracy. Does not apply to Command Panther.
Rank 3: Battle hardened crews improve vehicle mobility and rate of fire. Command aura now improves unit rate of fire. Does not apply to Command Panther."
In case these descriptions are correct, the panther does improve at vet 1-3.