Here's exactly what Clostermann said from his book-
"It was heavy, all of 7 tons. Thanks to its 2,400 HP engine it had a considerable margin of excess power and its acceleration was phenomenal. It was tricky to fly but its performance made up for it; at 3,000 feet, at economical cruising speed on 1/3rd power (950HP) with 2 45-gallon auxillary tanks, 310MPH on the clock ie., a true airspeed of 320MPH; at fast cruising speed, at 1/2 power (1,425HP) without auxillary tanks, 350MPH on the clock ie, a true airspeed of nearly 400MPH; maximum speed straight and level with +13 boost and 3,850 revs: 430MPH on the clock ie., a true air speed of 440MPH.
In emergencies you could over-boost it up to nearly 3,000HP and 4,000 revs, and the speed went up to 460MPH" and so on as you all have noted.
He never mentions 485 or 510 MPH as a maximum anywhere. He says he did 490 IAS once but at what altitude is unknown and it is unclear if he was in a shallow dive or not. What could be the blurry are is the difference in conversion of knots, IAS and TAS relative to the altitude. Clostermann being French thought in KPH too so did the editor assisting with "The Big Show" get any of the conversions right? Good question!
Clostermann is talking about speeds at 3,000 feet here and in one case with the drag of external tanks. We don't know the OAT -outside air temperature either, which affects things. Today we use a 2% factor as an average. Mathematically increase your indicated airspeed (IAS) by 2% per thousand feet of altitude to obtain the true airspeed (TAS).
Clostermann's conversions of IAS and TAS calculated at the 2% OAT factor come out a bit differently 310MPH IAS is 328MPH TAS, 350MPH IAS is 371 MPH TAS and 430MPH IAS is 456MPH TAS. A 1 % OAT factor would be 320, 360 and 443MPH TAS.
My P-51 manual shows a chart of the comparisons of IAS and TAS
at 40,000 ft.
IAS 260 MPH is 495 TAS
35,000 ft
290/500
30,000 ft
330/510
25,000 ft
360/520
20,000 ft
400/530
15,000 ft
440/540
10,000 ft
480/550
5,000 ft
IAS 505 MPH is 560 TAS
Since Clostermann is probably talking about the 1st Tempests in service we can assume it was the Tempest V (Tempest Is production plans were abandoned Tempest IIs entered service after the V) which had a top end of 435 MPH @ 17,000 ft.
The only 485 MPH airplane Hawker made was the Fury prototype with the 3,055 HP Napier Sabre VII 24-cyl flat H engine. This and the less powerfull 18 cylinder radial production aircraft were post war anyhow.
The V was the only legitimate Tempest in service in numbers. It was officially a low and medium interceptor fighter and fighter bomber hence the lower performance at altitude. This was the ship used to combat V-1s. Only 800 Tempest Vs ever existed with the total including post-war delivered VIs being 1,149.