How we can choice our Database block size and system block size ?

We use an Oracle database (8ko block size, 120Go) with 400 users for our X3 V6.

We will change to X3 U11 in few months. However we install the oracle for X3 U11 with 16 Ko for block size. We have a very bad test bench with this new intallation (MRP V6 : 60 minutes vs MRP U11 : 130 minutes ; the script you know ZX3IOBENCH.src give us for V6 a legacy time 46 and now with U11 we have a legacy time : 80). Have you an advice for the block size oracle/MSQL and block size system with X3 ?