Yes, I remember reading or hearing that in carbon steerer was introduced in 2014 frames.
Looks like CAAD and Allez use the same conical spacer http://www.tangeseiki.com/tangeseiki_ne ... 013293.gif
except height. CAAD is 25mm and Allez is 20mm.
Looks like 10mm conical spacer will be the closest thing with Bottom OD 46mm http://www.ebay.com/itm/8-10-15-20-25-m ... 0962175559
This is the first time I headr this: "you need at least a 5mm spacer above the stem to ensure it has a sufficient area to clamp on to so the force from the bolts is evenly distributed". I talked to mechanic about this...he insisted that 1-2mm spacer is required on top of stem, that's it.
5mm seems to be the most commonly used by pros, and is the most readily available low-stack spacer, so tends to be the go-to size. Theoretically a 4mm spacer would be just as good, but I think you'd struggle with only 1-2mm. I believe the recommended measurement is a 3mm drop between the top cap and the top of the steerer tube, so you'd ideally need something nearing 5mm for that reason.
As I said it's only a real problem with carbon steerers; if the stem doesn't have complete surface area to clamp onto it then the pressure from the bolts can deform the steerer. If you only had a 2mm spacer above, and had the recommended 3mm steerer drop below that top spacer then you wouldn't be getting the full security of the attachment. I wouldn't feel like the stem's doing its job properly if it wasn't using all of its clamping power where it should be.
On the other end of the spectrum, the stock stem on my Allez had a 4-position adjustment shim/plug which affected the angle of the clamping area and Specialized say with these stems the top cap must go directly on top with no spacer between.