The export preview of SVG is just a bitmap - zooming into it reveals this. This is not what it should do, and some will get confused when looking at the edges. I just got... well... you know. A bitmap with jagged edges is not at all a preview of a vector file so... hm.
Further zooming in with the keyboard is hit and miss in the dark with my non-uk keyboard layout, and the numeric keypad on a standard Apple keyboard is not supported - can't use the + and - keys there.
If I alter settings in the advanced section the preview is redrawn... at the original zoom level so Designer zooms out from for example 250% zoom to 60% zoom. Every single time I change a value!
And again zooming... you have to add UI control elements for zooming in and out.