Well, without even willing to get into the old UI discussion too much, to me all that wasted space of Metallurgy isn't compensated by whatever visual appeal.
Having said that, yes, the HX UI could be improved in *many* aspects. I certainly wouldn't mind a smaller, maybe even kinda photorealistic UI for some blocks (especially amps, cabs and dirt boxes), the main issue however not being the photorealism but the fact that HXN's sliders plain and simple just suck. Add to the general suckiness that controlling them with the mouse wheel is plain horror, getting all the way to the top of the nightmare podium when using a Macbook Trackpad (I actually don't know of any other plugin acting up that bad).
So, having said that, yes, from an "operational" POV, I can understand the appeal Metallurgy has (even if it's defenitely not because of the looks for me).
However, given VFM, HXN is an incredibly better deal. And I'm sure I can operate it as quickly as I could operate Metallurgy, even with all the mentioned UI quirks of HXN.