An interesting note is that in spite of all that power, the Do-335 was not the fastest piston aircraft deployed in WWII (474 mph), being outpaced by the P-51H (487 mph). I think it may have been the fastest to see combat, though, by the narrowest of margins (P-47M at 473 mph). It's worth noting that the current piston speed record lies with the single-prop, direct drive Rare Bear, at roughly 523 mph. (I've seen quotes that it could do 550 at altitude.
What this means for us is that the best speed is going to come from a direct drive, high speed, high pitched propeller on a single motor airplane. The only exception for this item would be in regards to aircraft which are sensitive to torque, which is why the propeller driven seaplane record set by the double-engine, contra-rotating prop Macchi MC.72 has stood for nearly 80 years and is unlikely to ever be broken.
