I used immediately your script and found the optimum on my San(StorageTek Flex 380) to be uniform size 25mb auto. It goes down to an avg DIFF of 18-20.

I am eager to see results from running this test on Solid state disk.

select table_name, column_name, securefile
from dba_lobs where table_name=’TEST_HW’
/
TABLE_NAME : TEST_HW
COLUMN_NAME : C1
SECUREFILE : NO

regarding improvement you saw with LOB column — is your database was set to use SecureFiles by default or you observe the same behavior with non-SecureFiles LOBs as well?