In regards to flap use by Mustang pilots:
"Kit" Carson of the 357th FG "If you're jumped, remember in the P-51 you've got (other than the Spitfire) the best defensive fighter in the business. Reef it in with full power and manoeuvring flaps ..."
Bob Welden of the 354th FG "... I finally got my sights on him from above by applying thirty degrees of flaps and full RPM."
Ed Heller of the 352nd FG "When I finally dumped flaps to decrease my turning radius ..."
Jim Starnes of the 339th FG "Since I was using maximum power and ten degrees of flaps to assist in my turn, I was easily outturning the FW-190 ahead of me."
Dick Asbury of the 363rd FG "I also lowered about twenty degrees of flaps. The use of flaps decreased the airplane's stalling speed and thus allowed greater maneuverability at lower speeds."
Bob Goebel of the 31st FG "Then I did something I didn't like to do: I put down 10 degrees of flaps. Putting the flaps down enabled me to turn tighter, but it reduced me speed."
"Bud" Anderson of the 357th FG "Instead of cobbing it like before and sailing on by him, I decide to turn hard left inside him, knowing that if I lose speed and don't make it I probably won't get home. I pull back on the throttle slightly, put down 10 degrees of flaps, and haul back on the stick just as hard as I can."
A lot of P-51 pilots used their flaps when needed. Some of these guys thought this was the greatest thing since sliced bread, and others preffered to just BnZ. The only information given in the P-51's pilot manual is airspeed limitations for flap useage. It doesn't give any flap G restrictions, and makes no mention of them in the prohibited maneuvers, accelerations limitations, or operating flight limits.
AH has it modeled correctly - they are there to use. The obvious drawback is that you have to slow down to do so which makes you vulnerable to attack.