The question you are asking is highly subjective and CAN get you a LOT of different answers and suggestions.
Are you after pure speed? Going with a smaller prop? Going with a higher pitch prop? Larger ESC? UBEC? Larger capacity batteries? Higher "C" rated batteries?
Are the "foam Mustangs" using retractable landing gear or fixed or are they not using any gear and being hand launched and belly landed. In other words are they "faster" than your Spitfire NOT because they have a larger motor but because they are LIGHTER AND SMALLER.
Want more speed? Take away the retracts totally. Cover the wheel wells with packing tape. Change the prop to the PZ extra 300 prop (it's a 10.5 X 9) and hand launch/belly land.
Generally speaking the "larger" motors are also lower kv motors and thus need to swing larger props to get the same speed as a higher kv motor swinging a smaller diameter prop. People choose the larger motors for that purpose so that they can swing a large 4 blade prop - more in line with 'SCALE" looks and not necessarily for performance. Some use the 2 blade props and bigger motor BUT add a cell to their battery - 4 cell. This adds the extra rpm to the smaller prop - but it is also adding weight.
As I said in the beginning of this long winded post - it is a VERY subjective question and the answer is really only known to you as only you know what you want from the plane in performance and looks.