A word on the Mustang. I have read this and that and personally I do not believe it's such an awesome plane. It performed well, was produced in large numbers and had insane range for that time. Most common US media will advertise it as uber-fast and uber-agile. From a pilots point of view, it's mostly advertised as reliable, comfortable and all around good. Plane performance is a subjective matter - raw charts do not cover real operating conditions, fuel quality, parts fatigue, weather etc. The Americans speak as highly of the P-51 as the Russians about La-7 and everyone else about the D-9.

I don't play DBW lately, but with 4.11 and the new AI and engine overheat, I'd say it's about right. If you fly properly and manage you pitch well, you can easily own a D-9. With the D-9, you can own a Mustang with a lot less effort (especially with the fire-power in mind since IL-2 doesn't give the .50 cal it's proper credit IMHO). I'd say it's fine.
I know that all the US plane fans will swallow the "P-51 was the best plane of WWII" propaganda, but you have to be reasonable about this. The allies won, they had more resources, had better teamwork tactics and were superior in numbers at the end of the war. The Mustang was good, but against what? Mostly outdated Me-109s built in a shed at the end of the war and flown be inexperienced pilots, because the Germans were running out of resources. I know with flight sims it's always the charts and veterans opinions, but war is more Starcraft than IL-2 - better economy and Zergrush grants the win, not one uber-unit. So if you ask me - the P-51 is OK.
By analogy, the popular statement says that the Spitfire was better than the 109 and Battle of Brittain proved this, however in IL-2 you can easily beat the Spit in a 109, if you do a lot of negative-g manoeuvring as the Spit's engine chokes during these and that correlates with historical data. On the other hand, in IL-2 you do not have bingo fuel after 3 minutes over the target and the enemy does not have radar advantage.
So just because the Mustang isn't a total ownage machine out of the box like the History Channel says, it does not mean it sucks. It just takes some effort to fly efficiently. Pro-tip from a not so pro pilot: 55% pitch unless in a climb really makes a difference. Oh, and the agility also depends on the prop pitch - with hight pitch , you're more likely to stall out while hard manoeuvring and a lot of rudder work is essential. Seriously do not complain, it's great plane, only it takes a lot of time to learn. I spent countless hours with it and I still feel like I have only scratched the surface. Perhaps it was easier for real pilots as they generally can properly feel the plane as compared to us flight-sim geeks, who only can rely on the visual aspect of flying. Anyway, stop complaining, it's not the plane - its the pilot.
Just my 3 cents.