The original WOP2 Mustang requires that you turn on auto mixture, otherwise you will need to set the mixture yourself when flying. It's more realistic having auto mixture with this plane enabled however. There is a third option. In your aircraft.cfg for your WOP2 Mustang, look for a line that says the following:
Change it to:
Now you won't have to select auto mixture from the realism settings. Now the performance charts will work and you'll get her up to very high altitudes. Please note that when the original WOP2 Mustang was developed, nobody had cracked the code for an authentically produced automatic carburetor. A2A was the first company to my knowledge to develop this, and do so properly. From the 377 onward, the carburetor works as intended and realistically, whereas before they were using the built in auto mixture system.