Internal diameter of MTB style bars varies considerably. In aluminum generally speaking the the more expensive the bar the thinner walled it is due to use of stronger grade aluminum and heat treatment given to the more expensive bars.

Not sure about CF bars. Unclear to me from your post as to whether the bar or brake levers are CF or both.

BTW seeing as how the external diameter of all MTB bars in the lever/shifter clamping area is the same, the OD at the stem has minimal influence on the ID at the end of the bar.