Read Greebo's post. This is not the same as adding the M from the D-40. There is no room on the skin bitmap for the D-11 to add the wing pylon.
This just means that the skins cannot be instantly transfered between the D11 and D23. In terms of the 3D mesh, this is much less work than creating a new plane, and so is the flight modeling.
The P47s line does need the most common Jug, and the best dogfighter of the bunch. Even the D11s that we have, spent much, if not most of their service with paddle blade props and wing mounts. If I am not mistaken, the paddle blade prop was introduced into the production line immediately after the D11 and quickly installed in all planes in the field.
For those who say that the P47s are over represented, this is just a game-history issue that gave us both D-25 and D-40, which is quite redundant. The D25 (as much as I like it) should have been a late razorback instead. However, if HTC do decide to model a late razorback, they will probably not remove the D-25 from the game, though I am perfectly fine with it if they do.