At macro distances wide apertures are difficult to work with. Even with a steady tripod 2.8 may not be enough depth of field. I tried doing this to get an eyeball shot and discovered that there wasn't enough depth of field for anything to appear in sharp focus.
Doing the focus and recompose, with the very shallow depth of field, will adjust your focus. Focus and recompose works in other situations because there should be enough depth of field that your minor movements keeps the subject in focus, but with the depth of field at macro distances and wide apertures that's not the case.
Also, the focus plane is flatter with that lens than others. Most lenses have a curved plane which, albeit slightly, helps with the focus and recompose technique, but with a flatter plane of focus it's slightly more likely to take your subject out of focus. I don't think it's a significant factor, but it's hurting, not helping.
When determining what an adequate shutter speed is, remember the 1/focal length rule (taking into account the 1.6 crop factor of your camera) and that the effectiveness of IS is advertised and may not be what you experience. If you have shaky hands you may need to adjust the rule to be a little faster. Also, I believe this only works for horizontal or vertical movements. If your DoF is so shallow your forward/backward sway may cause issues.
So, use a DoF calculator to determine the aperture you need which then determines the shutter speed you need, which then tells you if you need a tripod or not (I think most people find they need tripods)