This firmware update came out in August 2012 and is only required if the mark 2
300 2.8 lens serial number is between 790xxxxxxx to 913xxxxxxx
400 2.8 lens serial number is between 790xxxxxxx to 913xxxxxxx
500 f4 lens serial number is between 880xxxxxxx to 910xxxxxxx
600 f4 lens serial...
I have had a full frame camera (1dx) now for just over 2 months and have not touched my 1.6 (50d) or 1.3 (1d1v) sensor camera since.
It took me some time to get a full frame camera and my only regret is that
i never got it sooner.
I know the the 1Dx produces better images than the 1D mark 4 with my 600 f4 so it should be the same for a 300 and a 2x extender.
More pixels does not also make the image any better it is down to the quality of the sensor.
Full frame is the way to going
Just got a 1DX first time out with it and i am amazed by the images that it took on a very dull day and at iso that i would have never used with my old body and the images can be cropped and retain great detail
I have had my Wimberley version II since 15/12/2007 and it gets a lot of use in all sorts of weather and conditions.I too had this type of problem which i fixed by cleaning and lubricating thrust bearing in the lower part of the head.