"The Purpose of having WBS is for better cost monitoring, which with statistical wbs possible but, again we cant directly assign statistical WBS in SD Order as its required Internal Order Assignment." Are you account assigning an Internal Order to Sales Order line item? I don't understand.
If you are using valuated sales order stock where is the cost of sales being posted?
It doesn't make sense to use Project Systems if you are only using a single WBS element. The WBS hierarchy structure allows a more detailed cost segregation. But it doesn't sound like you are doing that.
You should be able to get the detailed reporting you need from the sales order.