The manufacturers of forks with cf steerers don't like you to use spacers higher than the diameter of the steerer. So for a 1 1/8" steerer, you don't want to use more than 2.8 cm of spacers. If you need your bars higher than this, you would use a stem that angles upward. But it's hard to get a flat stem (-17 degree rise, consistent with a 73 degree head tube) anymore these days anyway. Most stems now come with an 8 degree rise. I just hate this, since I prefer a flat stem, like in the old days.

Also, it's a good idea to always use a small (2 to 5mm) spacer above the stem before you apply the top cap. This acts like a washer, but also moves the top cap above the expander bolt ALWAYS used on a carbon steerer so that you are properly preloading the headset. Those expander bolts install right to the top of the steerer (unlike the star-fangled inserts used in metal steerer tubes), so you need a gap between the expander and the top cap.

That said, threadless headsets are such a HUGE improvement on the old threaded ones. Aside from the steerer tube no longer requiring threading (a big factor for shops, or if you've ever needed to get a steerer threaded - it actually requires a lathe; you don't just run a die over it!), installation is so much faster! When I convert my road fixie for track use, I even change the forks. This way, I don't have to disconnect the front brake (or the computer sensor), and it takes less than five minutes. Plus I no longer worry about the stem getting seized in the steerer tube after a winter of riding. Adjustment is dead simple: preload with the top cap, then tighten the stem to hold the adjustment. One 5mm allen key does it, unlike having to use two headset wrenches!

Luis