1) Can you test if zooming in while in 'No Filters' display mode (see menu 'View > Display mode') still causes such performance issues?
If performance is better (and the memory usage doesn't spike as much as with 'Normal' view) it would indicate that the renderer itself is the main cause - a solution will be available in the next major release (0.49) as described by ScislaC. In the mean-time - while using current stable 0.48.1 - consider toggling off the rendering of SVG filter effects (Blurs, all filter effects from the menu 'Filters' - preset or manually added in the filter editor) before zooming in very closely.
If you mainly use Gaussian blurs, it might be worth-while to test a current development snapshot build for Mac OS X . The formula for the rendering of Gaussian blurs was recently improved for better performance and might show a noticeable difference while still using the same renderer as current stable versions. Note though that development builds are usually not recommended to be used in a productive environment .
2) memory leak with embedded images: refers to
Bug #597689 “Memory leak when showing/hiding embedded images.”
While this can contribute to increased memory usage, it doesn't cause a sudden spike as you originally described. As far as I remember from a few tests, it is not triggered by linked bitmap images (when importing you can choose between link or embed, and later on you can embed or extract images with 'Extensions > Images…').
3) memory leak with pattern-filled objects: refers to
Bug #608944 “Enormous memory consumption (memory leak?)”
For this issue, unfortunately there is no known workaround to this date.
 You can download a DMG from <http://inkscape.modevia.com/macosx-snap/?C=M;O=D> and install it without deleting the stable version (install the development version into a different folder, and optionally rename it - e.g. Inkscape-rXXXXX - so that you know which version to launch).