Looking at the full size image on Flickr it appears that there is some subject blur, where even at 1/2000 the panning motion has not matched the movement of the plane. This is evident from the front fin at the bottom of the image where you can see blurring fore and aft of the fin, but very little at the edge of the tip. I don't know whether you used IS or not, but for shutter speeds over 1/500 its value is highly questionable and it may even do more harm than good. If you had IS enabled that might also account for the fore-aft blur. Turn it off for jets at 1/800 and faster. For choppers and prop planes at 1/80-1/250 then by all means use IS in the appropriate mode.
This blurring makes it difficult to conclude what else might be wrong, but I suspect some misfocus too, or general lens softness. At f/8 lens IQ ought not to be a problem, so I think maybe there is an AF calibration problem over great distance OR perhaps there is a UV filter fitted. If you have a filter fitted then lose it. The 100-400 does not like them. Use it in extreme conditions for protection, but not on a lovely sunny day when the lens is safe from flying debris, salt spray etc..
Personally I'd have halved the shutter speed and ISO for lower noise and relied upon better panning skills and luck to net me a crisp image to begin with. If you have a sharp image to work with then dealing with noise becomes easier too. If you have to deal with a soft/blurry image and noise at the same time you're going to struggle. Firing in short bursts should improve your chances of a sharper shot within the burst, but luck improves the more you practice.
Final comment - it looks like this was shot at quite a distance and on a hot, dry day the atmosphere can heat up and become turbulent (think heat haze) and can be full of dust and pollen which degrades IQ. If you can pick your positioning or time your shots for when the plane is closer then results should improve. Not only that, but being bigger in the frame means less cropping and that will also improve matters in terms of noise, detail and sharpness.