Hi there!
If you are using BI 4 you may connect to the BW application layer using a Multisource Unx, this uses the JCO connector, which has some things to take into consideration at design time.
If you are using any versions of BOBJ lower than BI4, you have to have Data Federator, which is a sepparate product, but it has the same design considerations as the imbedded DF in BI4.
Webi over Bex is so far away from being “optimal flexibility and performance” that it is just not worth the sarcasm brewing in me (@ Bex not @ Lugh). Using Bex as a semantic layer for Webi is technically possible, however, like everything else it has its limitations. 1st and foremost, do not expect to simply pick up a Bex query and expect Webi to do everything that Bex Explorer can do, or conversly, expect Bex semantic layer to do everything a Universe can do. It is very much like putting a Universe over a Cube, it does not take long at all to hit a wall.
Of course, unless you have Hana, you are not permitted to go directly to the underlying tables of BW with a universe at the database level, so the above are your only options, and both need some serious thought before ploughing ahead.
More info and links in these two threads…
https://bobj-board.org/t/202933
https://bobj-board.org/t/203835
Hope that helps.
coreya 
(BOB member since 2008-12-16)