Check on the ISO chart comparison.

http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=108&Camera=453&Sample=0&FLI=1&API= 0&LensComp=245&CameraComp=453&SampleComp=0&FLIComp =0&APIComp=0

 I am not seeing a better image quality here.

Thats not to say that yours didn't work out that way, it would just be unusual that a prime + extender would be sharper than bare prime.