A lot better almost seems like an understatement. North American Aviation had the opportunity of understanding the predecessors. They designed the intake do be aligned under the fuselage and the propeller specifically for this reason. The air flow coming from the prop alone helps significantly. The early Spitfires were barely adequate. When you are warming up sometime you may find your coolant to be very high while your oil temperature is still fairly low. You can open up your coolant door and rev up the prop a bit higher to get better airflow and you will see the affects that is has on your coolant temperature. I've done this before under certain circumstances to delay the coolant temperature rise while waiting for the oil temperature to get up. Just have to watch the oil pressure.
Interestingly enough the vent for the coolant system is present on the P-51 like it is with the Spitfire. The system is so much more adequate at cooling however that few people ever see that vent pop open.
I just made a similar test with the Spitfire MkIIb...
While coolant temp reached 120°+ and oil temp max needle deflection, the engine kept running nice.
Alot of coolant was lost through the vent -> 7% for those 20 mins.
This was with the coolerdoor fully open.
I think I must conclude that it isn´t unrealistic, but I should expect the engines to wear down quicker and for the Spitfire the amount of coolant will definitly sets a limit for how long it´s possible to run full power.
I can clearly see that the cooler system on the P-51 is alot better than on the Spitfire MkI-II.