This is more unstructured rambling than a conherent review...
I purchased a white pair of EU45 Empire SLX shoes in August of 2015 and have used them as my primary cycling shoe until last week. In the spirit of full disclosure, it's because the upper on my left shoe is coming unglued from the carbon outsole. It's a very small bit of separation, and I've temporarily mended it with Krazy Glue... I don't baby my shoes at all, so I'm inclined to believe this is an isolated case.
In terms of sizing, I typically wear US 10.5 dress shoes or US 11/11.5 athletic shoes. My feet are 273mm (10.7") long and 99mm (3.9") wide. This translates well to the Empire SLX in EU 45 sizing with a little less than a centimeter between my big toe and the end of the toebox. In many other brands like Shimano, fi'zi:k and Gaerne I usually go with EU 44.5. I have to wear 44 in Bontrager.
The overall fit is very good. With laces I can leave the forefoot/toebox a little looser while still getting the midfoot and heel nice and tight. This is somewhat necessary because the upper is very flexible. When I bought my shoes, they came with one pair of plain footbeds and also one pair of their SuperNatural footbeds with adjustable arch support. Both are a bit too thin for my tastes. I use fully custom orthotics with my shoes, but would settle for Specialized Body Geometry SL footbeds as an off-the-shelf option. The last on the Empire SLX is very flat otherwise.
Another thing I like about the Empires is how far back cleats can be mounted. I generally like my pedal spindles to be almost a centimeter behind the balls of my feet and this isn't possible with some shoes without adapters.
In terms of accessories, the shoes came with a fancy zippered, rectangular shoe bag, one extra pair of black laces, the SuperNatural footbeds and low profile titanium cleat bolts. The black laces were appreciated because eventually my white laces started to look a bit disgusting. After 1.5 years, my white shoes have yellowed a tiny bit. It's only noticeable when the shoes are placed next to something else truly white.
Performancewise, the outsole seemed plenty stiff. I don't think I'd be able to tell a difference between the stiffness of most high-end cycling shoes. All I know is that my power comes easiest in a shoe that fits. Ventilation is good, but my feet usually aren't bothered by feeling a little hot anyway. The shoes are very easily to wipe clean of grime and grease. And yeah these shoes are very light and that's something I did notice right away.
Would I go back to these shoes? Yes, definitely. I know they work well with my feet. It's just a matter of convenience and wanting to switch things up a bit. One weird thing I learned recently while shoe shopping is that the Giro Factor Techlace feels larger despite being built on the same last.