As modelled in AH, the planes are pretty evenly matched. The Dora holds the edge in acceleration and climb rate, and top speed at low alts. The P-51 has the edge in turning, low speed stability, and firepower. Neither plane can take a hit that well, I'd give the P-51 a little edge in the durability column though.
For the P-51 to win, he has to force the Dora to get manuever and get slow. Once the fight goes below 200 mph or so, the Dora is dogmeat. For the Dora to win, he has to force the P-51 to bleed off his speed to the point where the flaps come out, and then use acceleration and climb rate to wear the P-51 down.
With two decent pilots it will probably be a fairly long and evenly matched fight.