Here are some specifics:
You don't need to use WEP, just keep it at 95-100% power and 3000 RPM and it will have plenty of speed. All you will get for using WEP is an overheat. Leonard "Kit" Carson (18 kill Mustang Ace) said he very rarely used more than 55" MP (about 95% power)
The radiator is automatic and it will open up on you and slow you down at high power settings. Assign a key that will allow you to adjust the radiator and then close the radiator and you will go faster. At the above listed power settings you should not over heat very much. If you overheat, (usually due to hard maneuvering or steep climbing) then run your radiator selector to the Auto position again until you cool off for a bit.
Try to keep up your speed while climbing. The best rate of climb speed is 170mph, but that is way too slow. It will climb great at 200mph and you will have some energy for maneuvering if you need it.
You can dogfight with it, but be smooth. You don't need to yank until on the verge of a stall, you will find that it turns better if you let off the back pressure a bit, get out of all the stall buffeting and let it fly smoothly through the turn.
As far as the 190 being faster and climbing better at the medium altitudes, that's mainly because the P-51 supercharger is hasn't switched to high blower until you get above around 22,000 feet. Between about 10,000 feet and 22,000 feet you are in this area where the supercharger of the -51 is not at it's best performance and the 190 is in it's prime. Plus the 190 has better power to weight ratio and will accelerate better (another reason to keep your speed up) In these altitudes you will have to use lead turns (don't point directly at the guy you are chasing and expect to catch him) to close on the 190.
Above all, be smooth on the controls, if you jerk it into a turn it will be hard to get that speed back and you need it. It's your biggest advantage.
Oh yeah - again with the reduced fuel. 75% is enough, beyond that and you are not ready for combat. Another thing for success, don't just use those old 'quick combat' situations. Set up a simple bomber escort and fly 5000 feet above the bombers, then have the 190s come in at bomber altitude and you can dive down on them (A historically successful and realistic situation)
I could say more but blah blah blah.
