The P-47N was significantly superior in every area to the P-47D, and, I would argue, superior to the P-51D Mustangs that actually saw service.
From:
http://home.att.net/~historyzone/Seversky-Republic8.html
The XP-47N took to the air for the first time on July 22, 1944. Test comparisons were made with a P-47D-30-RE throughout the early portion of the evaluation period. Much to everyone's surprise, the XP-47N, with its greater wingspan and higher weight actually proved to have better roll performance than the D model. At 250 mph TAS, the N attained a maximum roll rate just over 100 degrees/second. The P-47D-30-RE could manage but 85 degrees/second at the same speed. At higher speeds, the N widened the gap further.
In mock combat with a P-47D-25-RE, the new fighter proved to be notably superior in every category of performance. In short, the XP-47 waxed the venerable D model regardless of who was piloting the older fighter. The new wing was part of this newfound dogfighting ability, however, the more powerful C series engine played a role too. The additional horsepower allowed the N to retain its energy better than the older Thunderbolt.
Perhaps the greatest performance increase was in maximum speed. Though not as fast as the stunning P-47M, the heavier N was fully 40 mph faster than the P-47D-25-RE and could generate speeds 30 mph greater than its principal rival, the Mustang. Scorching along at 467 mph @ 32,000 ft., the N could not be caught by any fighter in regular service with any air force on earth with the single exception of its M model sibling. This combination of wing and engine had pushed the N model up to the top rank of the superlative prop driven fighters then in existence.
The testing program included determining the maximum range of the fighter. This was done with various combinations of fuel loads and external drop tanks. Ultimately, a test flight was made from Farmingdale to Eglin Field in Florida. The XP-47N took off with two 315 gallon drop tanks hanging from the under-wing hardpoints. Usable fuel in these tanks totaled 600 gallons. Added to the internal fuel load, the N eased off the runway with 1,170 gallons of fuel (usable). At a gross weight of 20,166 lbs., the Thunderbolt headed south in company with a P-47D chase plane.
Arriving off the coast, east of Elgin in 3 hours, 44 minutes, the external tanks were dropped. Another P-47D, already waiting at Elgin, took on the N in a mock dogfight that lasted for twenty minutes. The throttle was advanced to military power for 15 minutes of this time, with an additional five minutes in the War Emergency Power (WEP) detent. After these fun and games were concluded the N was turned around and flown back towards Farmingdale. Heavy weather over Long Island caused the plane to divert to Woodbine, New Jersey. Having flown 1,980 miles, total fuel usage was measured at 1,057.5 gallons. There was still more than 112 gallons of usable fuel remaining in the main fuselage tank, enough for another 330 miles @ 1,700 rpm in auto-lean.
The XP-47N was now the king of long-range single engine fighters (the all time leader of long range escorts was the P-38L-1-LO, which could claim a combat radius of nearly 1,500 miles under ideal conditions).
It began service in the PTO in September of 1944. Total production exceeded 1,800 before the war's end.
From:
http://home.att.net/~jbaugher1/p47_13.html
The XP-47N flew for the first time on July 22, 1944. Such was the USAAF confidence in the Thunderbolt design that they went ahead and ordered 1900 P-47Ns in June 20, 1944, even before the first XP-47N had flown.
The P-47N was destined to be the last version of the Thunderbolt to be manufactured. The first P-47N-1-RE appeared in September of 1944, and 24 were delivered by year's end. The P-47N-5-RE and subsequent batches had zero-length rocket launchers added. The R-2800-77 engine was installed in late production models such as the P-47N-25-RE.
The P-47N gave excellent service in the Pacific in the last year of the War, particularly in escorting B-29 Superfortress bombers in raids on the Japanese mainland. P-47Ns were able to escort the bombers all the way from Saipan to Japan and on many other long, overwater flights.