sigma 70-200+ 2x TC vs sigma 50-500

[EYE_ON_ME]

Hi,
I have the sigma 50-500 and i use it to shoot at airshows and wildlife mainly zoo's, but some of the airshow photo's dont look very sharp. As the weather is getting nice and the airshow season starts i was wondering if the sigma 70-200 + 2x TC would be a better choice? afew of my photo's can be seen here:
http://www.flickr.com/photos/brendanwest/page2/
http://www.talkphotography.co.uk/forums/showthread.php?t=138801

 

[Les McLean]

Probably not, the 2.0 TC is not very highly regarded, unlike the siggy 70-200 F2.8 + 1.4TC, which is is a great combination

 

[trencheel303]

Don't use a 2x tc - it will reduce the quality of the image. All 2x do.

 

[mrgubby]

Never had much success with my Sigma 70-200 + 2x on moving targets

I'd guess that airshow photography could suffer badly from air distortion ? warm air over a distance can really screw with the shots so it may not be the 50-500 ?