Actually at one point I had a branch with a 1 step zooming functionality in the slideshow (basically a 2x zoom with a huge image download and 'follow-cursor' panning).

But I abandoned it to improve focus and simplify the UI.

Zooming on the client brings no additional details, so I doubt is very useful. Care to share a use-case for this?

Zooming in makes it easier to discern certain details. This is true, to a point, even if it increases the zoom beyond 1:1. This is increasingly true as the screen DPI get higher, and you can't make out individual pixels. Even zoomed in to 200%, a photo displayed on a "retina" iPad doesn't look jagged, it's just bigger. The same is true on a regular TFT viewed from a few feet away.

Apart from that, breaking browser zoom is just rude. It's a standard feature of the browser and apps/pages should never mess with it. I'm not saying you must have an application level zoom (e.g. by hooking the mwheel events), but breaking browser zoom is just not okay.