This is definitely an issue. I’ve run into the same thing and it has nothing to do with a compressor. When I add any of the limiters, FCX 10.1 treats -6db as zero. There are no compressors involved.
It seems like it might be an issue with compound clips. This is where I’m experiencing the issue. Were you running the limiter on a compound clip? When I tested this on one of the clips inside the compound it was not an issue.
The trick I’m using is to increase the audio level of the compound clip directly from the audio adjustment bar on the clip itself. You can increase the audio past -6db for every db you set over zero.
So for example, the audio bar on the compound clip is set at zero as default. With a limiter on the clip (any limiter will do, they all have the same effect) the audio will be stopped at -6db. Adjust the audio bar on the clip to +2db and you level will increase to -4db with the limiter active.
Definitely strange. Unless there is something I’m missing it appears to be a bug with compound clips.