If the given in-line grid does not serve the business needs of your organisation, it is possible to revert to CRM 2011 style where the opportunity product form opens on click of add.
You can create a new sub-grid for opportunity product and select the relevant view for the grid. This opens the opportunity product when + sign is clicked. Please note you should hide the other grid and not delete it just in case you want to revert back to it any point of time. This is because as it is inline editable grid, the behaviour of this grid is different than other grids.