Since you don't have any filter on the 100-400mm lens, my next suggestion would be to use the 70D's Micro Focus Adjustment feature to fine tune the focus with that lens (and any other lenses you might have, if you feel the need and wish to do so). There are instructions in the camera manual how to do MFA manually. There are various products available to help with the process (some can be homemade). There also are softwares that can largely automate MFA (Reikan FoCal, for example... Google it if interested). I think it very likely this would solve a lot of your sharpness issues once and for all.
I agree with previous responses that AI Servo isn't quite as accurate all the time, as One Shot can be. However, whenever you are shooting subjects that are moving... or that might move unexpectedly at any moment... AI Servo may be your only choice. (Live View can be even more accurate than One Shot, but also can only be used in certain situations.)
I don't have a 70D. I have a pair of 7D's. But I am fairly sure your 70D has a Custom Function settings much like my 7D's have, since they use very similar AF systems.
First, look for an AI Servo focus priority setting C.Fn. On my camera it's a single C.Fn that serves both 1st image (in a burst of shots) and 2nd/subsequent images (in the burst). I've set it to prioritize "focus vs. shutter release". In other words, I accept that the camera may shoot a little slower, may delay the release of the shutter for a fraction of a second to allow time for more accurate focus. I want my shots in focus, even if it slows down my shooting slightly.
I think your 70D might use two different C.Fns for this purpose... One for 1st image... The other for 2nd/subsequent shots in the burst. Some of the models newer than my 7D that I've looked into use two different C.Fn in this way. These C.Fn settings only effect AI Servo focus mode... not One Shot.
Using AI Servo a lot, you also might want to might want to experiment with Back Button Focusing. This is a very popular sports/action photography technique where you separate the focusing from the shutter release button. BBF allows you to start AF in advance of taking the shot, giving the camera a moment to acquire and track moving subjects. I've provided a link to an older article about BBF... on your 70D (and my 7D's) it's a little different setting it up. Our cameras already have BBF at the "AF On" button on the back. So it's really just a matter of disabling AF at the shutter release button with these camera models. Optionally, it's possible to swap the function of the AF On and "*" or AE Lock button (I use this because the */AE Lock button is larger, more prominent and closer to my thumb... and I focus every shot, while I only occasionally use AE Lock).
To set up BBF on your camera, you need to go into the button/control assignment and setup custom function and navigate to the shutter release button. There you set it to the second (middle) choice, which I believe is simply labelled "metering" or "start metering". That's all you really need to do, unless you want to swap the functions of the AF On and * buttons, which is done by reassigning them within that same custom function.
Try BBF and see if it helps. It quickly becomes second nature and puts you more in control of when and where the AF focuses. BBF also makes possible to use AI Servo by default (there are some situations or techniques, such as Focus & Recompose, that can't be done in AI Servo without BBF). Many people like and swear by BBF (including me). Some people just don't find it works for them, in which case it's very easy to switch back.
Finally, there are also custom functions controlling AI Servo tracking and how it deals with accelerating/decelerating subjects. Suggest you set the AI Servo tracking speed C.Fn to -1 or -2 at least initially. This governs how quick the camera is to jump to another object that might come between you and the subject or if you accidentally let the AF point slip off the subject momentarily. This C.Fn doesn't effect One Shot mode... only AI Servo. I find that when I'm in practice and better at keeping my AF point right on the subject, I can turn this back to 0 or even to +1 or +2 (which will deal with subject direction changes better). However I usually set this to -1 when I'm out of practice with the cameras and particular types of subjects.
The other C.Fn governs how AI Servo deals with subjects that are changing speeds. My cameras don't have this feature, but I believe yours does. Most people seem to find it works best for them set +1 or +2.
I hope these ideas help. One last thing I'd suggest is to set your camera to it's highest frame rate and get in the habit of shooting short bursts of 2, 3 or more shots, to best insure that at least one of them will be as sharp as possible. Better to have several to choose among, than none in focus. The above recommended settings and techniques, as well as the way AI Servo operates also favor using short bursts in this manner.
Oh, and while it's certainly commendable to try to "get it all right" in-camera as much as possible, IMO taking the shot is only half the job of making a photograph. Proper post-processing is every bit as important and essential to producing a great final image. I have never made a finished print or even shared an image online that hasn't been through at least some degree of post-processing . It might be only a few seconds to straighten, crop and tweak color and exposure levels (such as in Lightroom... for proofs). Or it might be many minutes or even some hours of work for finishing to a higher level (in Photoshop and similar... for prints and other purposes). By design, more advanced cameras (such as 70D vs T2i) do less image finishing in-camera, becuase more experienced users tend to do at least some post-processing. Back in the days of film, post-processing was typically done for us by our film lab. Today with digital, for all practical purposes we each are now our own "film lab".
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Alan Myers
San Jose, Calif., USA
"Walk softly and carry a big lens."
GEAR: 5DII, 7D(x2), 50D(x3), some other cameras, various lenses & accessories
FLICKR & EXPOSUREMANAGER
