Hi Ankit,
Check the point 6.1 in this document an approximation to your goal (Adapter Parallelism) http://www.sdn.sap.com/irj/scn/go/portal/prtroot/docs/library/uuid/f0b96675-87cf-2c10-b489-dee0ac03f782?QuickLink=index&…
I think Ankit you should talk with your basis team about to try to improve your PI performance, i think following the suggestions you can improve your PI and may be you don't need to do a drastic change of the messages to be processed at same time.
Regards.