Why can't you do some simple test?

Test -1 in a single user mode.
Assign a rollback segment.
With index delete some 100 rows.
From the v$rollstat you can figure out how much data has been written

Test -2 in a single user mode.
Drop indexes
Assign a new rollback segment.
Delete some 100 rows
From the v$rollstat you can figure out how much data has been written.

The diff between Test-1 and Test-2 is your answer.

Tamil