No, it won't affect how PowerStore works, but if you place a 'Slideshow Wide' gallery anywhere else, it will also have the new, narrower width.
If you have any doubts as to how it will appear, do a 'Save As' of the original javascript file (i.e. as slideshow_wide/gallery2.js) in case you want to return to it later, and close it. Make the changes to gallery.js.
Each of the gallery templates has its own javascript (gallery.js) file, so can't be affected by this change.
It would look like this with a width of 848px.