Performance Degrading After 11G Upgrade?

We have a strange performance issue in our production database.

After migrating our production database from 10g (R2) to 11g (R2) we started seeing major performance issues in the I/O's.

we do have baseline calculation process which runs past midnight every day. Before 11G migration it used to complete in less than 1 Hour, since migration (3 days ago) it started taking more than 5 hours. It just showing major I/O activity (db sequential reads) when the job was running but nothing else. The I/O was always because of the index range scan over a 1.8 billion rows table. By using the Hints we forced Oracle to use the same explain plan it was using on 10G. The query basically index range scans last 60 days worth of data for a specific person out of 1.8 billion rows. Indexes are created with date column in descending order to make the index scans faster while searching for latest data.

when i compared the historical AWR from 10g to 11G I have noticed for same process number of physical reads(sequential) are more in 11G than in 10G. Even physical I/O waits are also more comparatively in 11G because of the more reads i guess. I am thinking may be Oracle scanning index more than what it needs to though we have Histograms on the indexed columns.

My question is , are there any other factors/parameters that we can manipulate to cheat Oracle and make it do fewer physical reads (required) for both both index and table.

Does Histograms only selecting correct index or do they dictating how much percentage of index to be scanned for the current operation also?

Again indexes are same and explain plan is exactly same but more physical I/O in 11G than in 10G. Way to minimize the I/O scans?