conditional unique index to restrict duplicates of a particular set

I have a few questions with reference to below scenario:

Table Structure : feed_details( id_pk      number,
                acc_number number,
                                branch_code number,
                                feed_date   date,
                                feed_type   varchar2(1),
                                balance     number
                               );

The feed_type can have three values 'I','E' and 'P'. We have to put a conditional unique constraint on acc_number, branch_code, feed_date and feed_type. The feed_date will have date and time will be always set to midnight.

The constraint is to be put such that duplicates of feed type 'P' should be allowed but duplicates for 'I' and 'E' should not be allowed.

e.g. acc_number|branch_code|feed_type|feed_date
     101       |10         | I      | 07052014
     102       |10         | E     | 07052014
     101       |10         | I       | 07052014 -- should not be allowed
     102       |10         | E      | 07052014 -- should not be allowed
     103       |10         | P      | 07052014
     103       |10         | P      | 07052014

FB unique index as below would allow this to be achieved.

CREATE UNIQUE INDEX idx_feed_details

    ON feed_details(CASE WHEN fed_typ!='P' THEN feed_date ELSE null END,

               CASE WHEN fed_typ!='P' THEN acc_number ELSE null END,
               CASE WHEN fed_typ!='P' THEN branch_code ELSE null END,
               CASE WHEN fed_typ!='P' THEN feed_type ELSE null END);

1. How should we correctly measure the performance hit after his index is created (the inserts would be concurrent from multiple java connections).
2. Since the index is only to restrict duplicates, would this index be helpful in some other scenarios of data selection when the predicate is on these columns.
3. any other overheads / side effects of this kind of index.

Hope the questions and scenario is clear.

Thanking you all for your time and help.

Warm Regards,
Vijay Sehgal.

--
http://www.freelists.org/webpage/oracle-l