Canon's extenders are the best there are, but they only work with L series telephotos of 135mm and above, and most L series tele zooms except the 70-300 L IS. So basically they're no help to you. Cheaper teleconverters are not as good optically, so won't give you a great result, particularly if you pair them with cheaper zooms - the quality really does degrade. As a rule of thumb, always use the lowest-power extender/converter you can - i.e. a 1.4x for best quality.
i would say its not to bad using this Sigma lens with the 2x converter took this at my local airfield
but your right Canon is the best but until i win the lotto this one will have to do