It really depends on which P47D variant we're talking about when it comes to it's maximum speed. More-over, it depends on which enigine was installed.
The P47 was devoloped by Republic Aviation's chief engineer Alexander Kartveli and was equipped with the Pratt and Whitney R2800 an 18 cylinder, Double Wasp, radial air cooled engine producing 2000 HP. The R2800 needed a highly efficiant duct system for its supercharger. This requirement led to the shape of the P47 as we know it now. The R2800 had many variants which produced more an more power as the war progressed
As quoted from "P-47 Thunderbolt: Aviation Darwinism"
As the German army was pushed closer to the border of their homeland, newer sub-models of the P-47D found their way into service. The P-47D-27-RE arrived with an up-rated engine of 2,430 hp. and Hamilton-Standard propeller. The P-47D-28-RA was essentially the same aircraft, but used a Curtiss Electric prop. Some of the next to last sub-model, the P-47D-30-RE, incorporated a new dorsal fillet at the base of the rudder assembly. Virtually all of the final D model, the P-47D-40-RA had the fillet installed. The loss of side area due to the cutting down of the fuselage (to accommodate the bubble canopy), reduced the linear stability of the bubble topped Jugs. This could lead to a loss of control if rudder deflection was too great at low speeds. The fillet partially corrected this rudder force over-balance condition. It would not be fully eliminated until the final production model of the Thunderbolt. The P-47D-40-RA was the first P-47 to have the new K-14 gunsight installed at the factory. It was also the first Thunderbolt to carry the new tail warning radar equipment.
The P-47M series used the R2800-C engine, and was the fastest propeller driven aircraft to see service over Europe with an "official" speed of 470mph at 34,200 ft. There were only about 120 of these models to see service. It's really just a co-incidence that the P-47M was in development while V-1 rockets were raining down on London. The R2800-C engine was already in development before that time. Actually, by the time the plane saw operation on 3-Jan-45, most of the V-1 site had been over-run by allied forces.
The M series did encounter some troubles during flight with the R2800-C engine. Pilots were experiencing misfirings, inability to extract maximum power at altitude, and one pilot suffered complete engine failure at 32,000 ft. These problems turned out to be caused by the ignition leads. The problem of max power was found to be due to the correlation of the throttle and turbo-supercharger settings.
The P-47N series was in development toward the end of the war in Europe, but wasn't in service till after VE day. The main roll of the P-47N was utilized in the Pacific Operations over Japan escorting our B-29's.
The P-47N could fly farther then it's counter-part in the ETO as well as having the distinctive "clipped-wing" design. It's interesting to note that the wing was actually 14" longer then the P-47M and throughout testing performed a much better roll rate. There were approx 1800 of the P-47N variant to serve before the end of the project.
If you'd like to learn the history of the P47:
P-47 Thunderbolt: Aviation Darwinism
