The reciprocal rule, as you call it, is nothing but a guideline or rule of thumb for the
minimum shutter speed where one might reasonably expect to get reasonably sharp images with good technique. IOW, go below that guideline, or don't use very good technique, and you can expect your images to be blurred. Beyond that, blurred is a relative characteristic. If you're looking for critical sharpness that holds up at 100% views on your monitor, 1/focal length is not where you want to live. Add to that the fact that IS adding three stops is also a guideline that's usually qualified with terms like "up to," and you're out on the bleeding edge of what can reasonably be expected.
In any event, mirror slap is highly unlikely to be a factor when handholding. It's a factor when you've eliminated everything else — i.e., you're on a very good tripod, in windless and vibration-free conditions, using a remote release.
A heavier camera might help on the margin, but it can cut the other way when fatigue sets in. Look instead to higher shutter speeds and better handholding technique — good stance, holding the camera correctly with elbows tucked in and braced against your body, squeezing the shutter gently as you exhale, etc.
Nill
~~
www.toulme.net