discharge

I'm trying to figure out the threshold amount of interest/fees savings in order to make it worthwhile to move lenders (refi). Say I have a $250k loan PPOR no LMI 80% LVR (plain vanilla type of loan), and want to refinance, how much interest and fee savings would it take for it to be worthwhile...

We've paid off our house and want to invest in a property. We have the money sitting in an offset account and haven't discharged the mortgage on our home yet. Should we borrow for IP from the same bank? Is there any benefit to discharging the mortgage (security?)? Thanks.