If you keep the speed up then its not only possible to run until the fuel runs out but also to fight for very long periods of time on full power in the Spit 25lbs. And it is very easy to keep up speed on the Spit 25lbs, because you have just over 2000hp hauling a plane that is by late war standards very light.

Here is a scary piece of info for the Blue pilot out there :)

25lbs was not the most boost that a Merlin 66 could be made to use, there were also successful experiments with 28lbs boost giving almost 2200hp.

Of course, by the time this could have been made operational, there was no need, the Allies had air superiority, most Spitfire IXs were being used for ground attack, and even the 25lbs IXs were changed back to 18lbs to standardise fuel types and reduce engine wear, but it is still something to think about.