At low altitudes the Merlin P-51 is slow and weak.
The P-51B/D are heavily optimized for extremely long range missions, at 30,000 ft, with high cruising speeds, and excusions in the transonic flight region. The Il-2 flight engine was optimized for a study sim of the Il-2, a plane which barely goes above 10,000 ft, and almost breaks 300 mph. Above 15,000 ft, many of the flight model assumptions start to break down.
Additionally, in online dogfight, most fights take place in the 0-5,000 ft range, simply because climbing to 30,000 at a sustained climb rate of 2,000 fpm takes about 15 minutes, and people really aren't into spending a quarter of an hour just climbing to altitude, when they want to get some gun-fight on.
In the 0-5k ft flight regime, in the limits of the Il-2 engine, the P-51B/D models have significant issues, because of the trades that were made for it's exceptional high altitude performance. The first is the 2-stage, 2-speed supercharger. It's a marvelous supercharger, with a critical altitude of something on the order of 26,000 ft. To do that, however, it pulls about 100-150 hp out of the engine, even when it wasn't delivering maximum boost. For Sea Level power at 8km, it's worth it, if you're at 8km. If you're at sea level, it's lost power.
The next issue is fuel. The P-51B/D has a range of something on the order of 2,000 miles. To get this, the designers crammed fuel tanks into every possible orifice one could fit. The specific culprit here is an 85 gallion (about 500lbs) that was added being the pilot, with no counter balance at all, starting about the P-51B-7 model. If this tank is full, it destroyes the P-51's stability; try to do anything more exciting than flying in a straight line, and you'll spin. The hazards were justified by the range the tank added, and were mitigated by operation paramaters which involved 30m to an hour of flight simply spent getting to altitiude and linking up before cross the lines, in which this fuel tank is drained, and stability restored.
This is where one of the Il-2 engine limitations comes in. None of the fighters originally modeled had complex multi-tank fuel systems, and none of the ones even being considered have anything like the complexity involved in the P-51's unbalanced fuel system, so, for simplicity, they built an engine that only models a single fuel tank, and if I'm understanding the way its said to be set up, we're talking about a linear slider on the CoG that moves depending on how full the tank is. The game engine just doesn't have the hooks to model the screwy P-51 tank layout, without either draining all tanks evenly, or deleting the 85 gallon tank from the plane, cutting the total fuel load by about a third. Neither was very palatable.
The final limitation is the laminar flow wing. Laminar flow wings are very good in the 350-500 mph range, and, as I understand it, help delay the onset of shock wave drag. I don't believe we have shock wave drag in Il-2, or any of the funky transonic effect you have when you get above 500mph, or the way those fun effects seem to occur at low IAS the higher you get, which is, I believe part of the reason the flight model starts getting odd at high altitudes. So the big advange of laminar flow can't really be modeled in this game. However, the main draw back of the laminar flow wing is very much a subsonic phenomina: they have a high stall speed, and poor lift. This mean lower over all turn rates, and a tendacy to snap stall. Combine that with an engine that's spending a large chunk of its power on something you aren't using, and a fuel tank that's disrupting the CoG, and you're going to have some real manuver problems.
One other aspect that limits its instant dogfight potential is the 0.50's. 4-6 0.50 caliber machines guns just don't pack the stopping power of quad 20mm or 30mm cannon. This was another ConOps driven trade, primarly by the need of USAAF aircraft to be in enemy airspace for 5-8 hours at a time. 60 rounds of MK108 will ruin somebody's day; one somebody, and then you get spend the next four hours hoping nobody notices you. 600 rounds of MK 108 will ruin a lot of people's days. It also weighs 600lbs. For that much mass you can put nearly 2,600 0.50's on you plane and shoot all day. They won't turn targets into the collection of confetti that the 108 will, but they're effective against the armour that most fighters of the time were limited to, and the lightly armoured ones got chewed. That being said, I suspect the USAAF wouldn't have been to upset with a 1xMK108 version that did come with 600 rounds, though I would have loved to see how NAA would have fit that keg-o-ammo in.
Given the wall of text above, I'm of the opinion that the only Mustangs that would perform well in the current engine are the
Mustang I/IA that the British used in the 1942 time frame. They were Allison engined, low altitude oriented aircraft. They didn't have the 85 gallon fuselage tank, and the MK IA was armed with 4x20mm Hispano cannons, so they had high alpha that quick action requires. On the other hand, right now Maddox is currently fully engaged in developing
Storm of War: Battle of Britain and any time spent on adding new planes to the old engine is time not being spent making the new engine, which is intended to fix most of the issues that high speed, high altitude aircraft have in the current Il-2 engine.
So yeah. Welcome to the Il-2 flight model debates. Check your life at the door.