I think that not only does mounting make a difference - but the type and size and colour of the mount matters, too.

I entered some prints into a competition. They were black and white 16 X 12". I spent quite some time with in the local art shop with the guy who was going to mount them, who was an artist, discussing and trying different sizes and colours of mounts. We settled on a 2 inch mount in chocolate brown with the edges painted (rather than left white). This seemed to lift the highlights and make them glow more than a pale mount would - but a black mount seemed to make the shadows in the pictures seem less impressive.

I liked the effect, I felt it added a lot to the images.

I got some sort of prize in the competition (nothing special) and my prints were displayed, along with the other winners and commended entries, in a local gallery. Without my permission the organisers had decided to mount every entry the same way - in a huge white mount. IMHO It made the pictures look terrible.
I was furious. Never entered a competition since :-(