If, however you're talking about 1 1/8" steerer then the full Syntace range is available in both 25.4mm and 26.0mm. I think they used to be available in 1" steerer and also with a zero-rise BITD The lightest of the lot, the F99 is here
I have a 90mm one on my road bike - it's actually a +/-6 degree stem, but pretty subtle on such a short stem, you really don't notice it. I might consider selling it on in fact - it's probably absolutely fine but I just don't feel 100% comfortable riding over Flemish cobbles with such a light stem. I'd never ride around here with a carbon stem for that reason.
They recommend only using it with the correct steerer size but that sounds like an exercise in litigation avoidance to me. I can't for the life of me see how using a shim could lead to problems through anything other than bad installation.