You said you reinstalled drivers when putting the old card in. Did you remove those drivers with DDU or AMD Cleanup Utility before reinstalling the new card, then install drivers for the new card?
If so, are you on the same driver as before?
Did you previously apply any overclock or settings tweak in your software or BIOS?
Does the card have one or two PCIE power slots? If two, are both plugged firmly into power on the card and your PSU?
Did the game itself do any kind of auto detect of the card and apply a different default setting that you had previously changed manually?
Were there any Windows updates that installed in the interim?
Did the game update to a newer version that may have affected performance? Check their forums and see if there are any other users with the same or similar card complaining.