There is a sample script for XOGing in
There is a logic between the project object and the task object among other things. Updating just inv_investment table only is likely to break that logic.
When you XOG in the project data to set the start date you would be better off to set the the start date of the first task as well as they are connected. If you have resources assigned and ETC applied that point it gets even more complicated, but still doable.
You would launch the process on create to change the start date so you would be XOGing a project which is already created and so you would only need minimum data for the xml file.