I know that 'my' copy of the 100-400 severely dislikes filters; I'll get relatIvely soft results from it even with a high end UV filter.
The other thing is that the IS system in it is old, and to really get sharp results handheld you still should try and keep the shutter up around 1/focal length. At 400mm, I get the best results at 1/640 or higher. That isn't to say that great results can't come at slower speeds, but it is harder to get them.
If you're concerned about sharpness, take some shots using a tripod, a remote shutter cable/timer and mirror lockup. This will remove jitter from you and from mirror slap.
Your example, though, looks like you just had a shutter slightly too slow or that you 'stabbed' the shutter button instead of a smooth press.