When I first tested this against the Mk2 extender for IQ with static indoor test subjects, I thought I had made a mistake in spending so much in that the differences in IQ other than in the mid / corners did not seem that different.
Cost effective solution as works superbly with my 70-200 2.8 Mk11; much better contrast / saturation and tighter fit than Mk11 version
Images are slightly degraded and will always be compared to equivalent length alternative lenses