The answer to your question is; No you’re not being unrealistic to expect more sharpness from this lens. This is an L lens and they do not get any better for such Zoom Lenses. I am giving my opinion based on experience.
I have found that a lot of Zoom lenses do not work straight of the box and need tweaking. I have had to tweak at least 90% of my lenses since adopting the digital scene to increase sharpness.
I apologise up front if my answer covers anything you may have already looked at, but for this particular lens, which I also use on occasions, the first thing that I do before taking a single shot; is to check the focusing.
I keep it very simple and and use the box that the lens came in, to do a micro-adjust and correctly calibrate. In the past, I always used to send the lens back for calibration but now it is so easy.
The issue is that Camera/Lens Manufacturers produce their Cameras/lenses within a tolerance range and sometimes, it is possible to fall at the extremes of both limits.
For example, say the Lens is +5 out and the Camera is -5, you will be fine and have a perfect focus, likewise, if your camera is +5 but the lens happens to be -5, you still get a sharp focus. you are non the wiser if either of the variables are accurate or not.
The problem comes to light, if the Camera is 100% Accurate and the lens is out. or if the camera is + 5 and the Lens is also +5 or +3 or whatever.
This issue causes the lens/Camera combination to either focus slightly before(back Focus) the intended subject or slightly after (front Focus), depending on the plus minus figures.
To correct this, it is a fairly easy job and can be done for each different lens which is auto saved and readjusts itself each time the lenses are switched.
Go into your MENU and find AUTO FOCUS DRIVE and select AF/MICRO ADJUST.
There are many techie ways of doing this, However, I just use the Lens Box. I place it on a sturdy surface with Blutac.
Place my Camera on a sturdy Tripod between 10-20 Feet Distance. It can be more, makes no difference.
I make sure that the box is at a slight angle to the camera, about a 30 Degree Turned to the right.
In Autofocus mode, I use a single Focus point, press the manual zoom button to get a closer look and take a shot of a letter somewhere in the middle of the line.
I then go into the Micro Adjust and increase by +1 and take another shot and I keep doing this until I reach the end.
I then reverse the process and go the other way. Generally, about 20 shots. ( you don’t have to take this many, but its just me being anal!)
Once done, it is time to compare each shot on the computer. By having the box at an angle, you will be able to tell if the camera is back/front focusing and be able to determine, which of the 20 shots is the most in focus.
Ensuring that you have kept a note of the plus/Minus, that is the setting you need to adjust at.
Sometimes, you may find that the same lens will have slightly different adjust at the different ends of the focal lengths, doesn’t happen that often, but I do like to check that too.
if this doesn’t work, then of course, you also have to consider other variables such as wind. the speed you have used of 100sec may be too slow ( just a thought). The depth of field will also make a difference. you may also consider disabling the Highlight Tone Priority in your camera.
Hope this help, and please do let us know how you get on. Good luck