A pdf Spitfire V manual says it cruises at 180 mph below 8,000 feet; 160 mph from 8,000 to 15,000 feet; and 150 mph above 15,000 feet. At very LOW altitude, can increase to 200 mph without seriously affecting range. Speeds are I.A.S.
On a typical day, 180 mph IAS at 8,000 feet is about 209 mph or so TAS.
So, a Spitfire V would have difficulty keeping up with an A-36 at 250 mph without having to abandon the escort before long. It had notoriously short legs.