Can't see a whole lot of difference other than aesthetics in most of the shimano pedal range
You clearly haven't looked very hard then!
The 540's have a relaitvely narrow profile, like a look Keo Easy. The 105 5700's and above are all much wider, creating a huge platform to stand on and really alleviate hotspots.
Proper road clipless pedals do make a difference, it's just how much of a difference is the question. If you're serious about road riding, then without doubt you'll want some and a stiff pair of shoes to go with them. If like me, then I can't abide having to walk anywhere in roadie shoes with the cleats hanging off the bottom, and I've already got 3 perfectly good pairs of MTB clipless shoes anyway. That and one of my pairs of shoes is a pair of MW80 winter boots, which in this country stay on my feet about 6 months of the year anyway, and I AM NOT putting some lightweight vented roadie shoes (even with some overshoes on) to get cold and wet feet for anyone!
I break a whole heap of the Velominati rules when I ride my road bike, and I can live with it... Whilst I'm never the fastest in the groups I ride with, I'm not the slowest either.
But back on topic... If you do suffer with hotspots, that's the biggest difference a pair of proper road pedals and shoes will help out with.