Jim,
Without knowing your exact end-game, I cannot recommend a proper workflow for you. With that said, here is what I believe your seeing. When you select to render out your comp to the Media 100 HD codec, your resulting file is going to be 10-bit YUV. When you then re-import that file into AE you are seeing a 8-bit RGB image…Here is why…
The way the Media 100 HD codec is written allows applications to specify what mode the codec needs to be in (we call this interfaces). There is the YUV mode and the RGB mode. Since AE is an RGB application your 10-bit YUV image is being used in 8-bit RGB mode (or the 8-bit RGB interface) within AE. The actual source file is still 10-bit YUV but being displayed as 8-bit RGB.
You could always experiment with different codecs to get a better result. Other codecs may expose you to different YUV/RGB interfaces and give the proper results. You are never required to work in the Media 100 HD codec within our application so feel free to run some tests.
I hope this helps you.
Rob N.