Getting the highest performance from a lens

Canon 7d, 100-400, 400mm F5.6, ISO 400, 5.2Mb file (Crop of a Croc)

Okay, I will admit it, I have sharpness envy. I was quite happy with the photos I recently took on a safari with my 7d/100-400 mm combo (with monopod) until I started looking around and saw just how sharp this lens can be. Now, this was the first opportunity to spend "quality" time with the lens and as you can see, the results could be quite sharp, but still not as sharp as I've seen, despite my best efforts. Part of the problem with safaris is you are usually shooting in low light, hence the use of F5.6 most of the time when at 400mm.

My question is: Is it better to increase the ISO to a point to enable the use of this lens a couple of stops from wide open (say f7.1) or will the results be better shooting wide open at the lowest ISO possible, especially when there is potentially the need to crop the photo, as was the case above.

I don't think micro adjustment of the lens would be my first route to sharper photos so looking for any and all thoughts on this perplexing matter! Thanks.