So perhaps, like you said, our solution would be to lower the Corsair's maximum throttle setting?
I think the reason as to why there is no time limit stated for max take-off is because take-off isn't really going to take a significant amount of time to execute, thereby removing the necessity of implying a time limit on the take-off setting. But looking at all the figures you posted, it is indeed true that the P-51B, F4U's and probably some other planes are using higher MAPs and RPMs versus the Spitfires.
The strange thing is that the US manual states different times for which the F4U can use 2700 RPM/53" Hg. Regardless, I tested the F4U at sea level at ~ 2500 RPM/49" Hg (it's hard to get the extra 50 RPM/0.5") and it turns out that at this setting, the F4U-1A is 3 mph faster than a Spitfire VIII on WEP. Maybe Sax will be slightly comforted by that fact. I know I am!
EDIT: I just can't keep my head from thinking after I finished the post...
It seems that HTC has simply modeled each aircraft's engine based on the absolute max throttle settings. Let's disregard the normal, climb and cruise settngs for now. The absolute max throttle settings for the F4U-1A are 2700 RPM/57.5" Hg in this game. The charts say that the absolute max settings are 2700 RPM/60" Hg. Let's say that setting is equivalent to the 2700 RPM/57.5" Hg we have in-game. The only lower setting is that of the setting without WEP on the charts posted: 2700 RPM/~53-54" Hg. That becomes the maximum military power setting and is continuous.
In the case of the SPit XVI, we have 3000 RPM/+18 lb for absolute max. The take-off happens to be the same. Therefore, the engine settings that are immediately lower than this are 2850 RPM/+16 lb. That setting then becomes MIL and is continuous.
If my assumptions on this engne model is correct, all HTC would have to do is add the extra RPMs to the Spitfire IX's MIL/combat setting. I know this is not completely correct, but it seems as though HTC has tried to set up a compromise between what we would see as 2-stage WEP and a simple engine model by only applying a 5 min limit on the absolute max settings to engines of ALL planes.