I'm thinking that this would be a great idea. You already have the internal engine sounds, so why not use external engine sounds as well?
It would be a great effect for the runways, as well as flyby's. A regular engine sound is labeled eng, and an external one could be labeled exeng.
Along with external sounds, we need more engine variations. Low RPM ratings sound much different from high RPM ratings. Such as 4 RPM variations, like LowRPM, MidRPM, HighRPM and WEP.
It wouldn't take much to do these things, and it would add that extra dimension of depth.