We are in a similar situation considering the add-on.
With regard to the banking deadlines, remember that the number of points you can bank includes the total amount of points in your account. So, for example, if you have 200 points now and you add 120, all your banking numbers would be based on the full 320 points. If you are within the period where you can bank 50%, you would be able to bank a cumulative total of 160, not just half of the 120.
Also, I have been told that the developer points are not bankable. Depending on your use year and whether you can get (or want) '04 points, that may or may not be an issue for you. If you use the developer points and bank an extra 100 of your regular points, there is no effective difference, but if you're getting points you can't use, it will make a difference.
We have an October use year. In our case, we can get '04 points, which means the developer points would also be '04 points and if they're not bankable, they will expire on Sept 30, 2005.
We have already done some banking, so some of our add-on points will be bankable and some will not, meaning we will have both developer points and some of the add-on points we have to either use, rent, or lose by 9/30. We will just rent those to defray some of the cost of the add-on.