If all the new players jumped in a D9 or PJ, they would have terrible k/d ratios.
For the P-38 this is no doubt true. For the D9, this is false, because the D9 does 90% of what a P-51 does but does it better. Better top speed at typical MA alts, much better climb and accel, better roll rate. What the P-51 does better than the D9 basically comes down to turning, but as this is not a great strength of the P-51 vs. anything *except* a Fw, this is not such a great matter. If the P-51 did not have the "name" the Dora's usage in the same role would be greater.