Unfortunately, the only true roll-up supported is one created at the Work Item level. This field will continue to roll-up to the project portfolios; however, the field must be populated at the leaf level (usually a task). I will add you to the feature request for the ability to set the non-leaf fields values (similar to revenue fields, for example). For your project level fields, another option is to create a formula roll-up summary field at the project level which sums the "Impacting and Sub Items" field value. This will create a separate field on the object for roll-up purposes.