You have to remember that the P-51 is primarily a high altitude, long range bomber escort. This is where it excels. If you want better climb performance, take less fuel. The Mustang holds a whole lot more than the Dora and in a short range, low altitude fight you won't need it. The Dora is a monster low down and was improved to counter higher altitude threats like the P-51, but it's at its best at low and medium altitudes.
I don't feel the aircraft need balancing at all. They both have their advantages and disadvantages just as the real aircraft did. Sound tactics in either aircraft should be enough to triumph over the other at the end of the day.