I might be wrong about this but surely the question of whether it is better to overpay on a tracker or put your money in a savings account is dependent on time.
If we assume that you have a tracker mortgage costing 1.5% and a savings account that pays 2.5%. And that you have 1000 euro and you are trying to decide what to do with it. If you put it in the savings account you will make 2.5% a year on it. If you pay it off you mortgage you will save yourself 1.5% a year. So on the face of it, it seems like putting your money in the savings account seems best.
However, surely you will only make the 2.5% on your savings for the time that the money is in the savings account, whereas you would make the 1.5% saving on the mortgage for the rest of the term of your mortgage. On the assumption that the remaining term of your mortgage is probably a lot longer than the amount of time you will keep the money in your savings account, then the option to pay it off the mortgage will save you more money in the long term.