Does the CMAN version of berserk stack with the guild version?
If so, what are the tangible benefits to such an investment?

The benefits of training in Berserk via the Guild are essentially two:

1) Mastery of berserk via the guild is supposed to be like having 6.3 CMAN ranks (having 50 ranks by the guild is supposed to equal 5 ranks by CMAN).

2) You free up the CMAN points you would have used to train in berserk to train in something else. The same with tackle, disarm, feint, etc...the guild skills mean you don't have to waste your precious CMAN points on those skills.

-Mohrgan

The general rule of thumb is 1 CMan rank - 10 guild ranks, but its an approximation and there are variations between skills in how the CMan is coded viz a viz the guild skill. If you are interested in a particular effect that is triggered at a rank threshold (like barehanded use) that varies quite a lot.