It all depends on what you do with that extra money every month. If it is your intention to save that much every month every year and will not need any of it again then I would say the offset is a waste of time for you. You could get a better tracker mtg rate and just pay the extra into that and it would finish early just the same.
However if you want the flexibility of maybe not putting in the extra money every month or you will need to draw some of the spare money back out again then the fact that it is not in your mortgage but just in a linked savings account makes that easy.
The rate on the offset is 5.15, with your LTV you could probably get a tracker of 4.5ish. You could then if you wanted lodge the extra into the FA esavings account at 5.22% or whatever high rate account you can get and transfer a lump sum into the mortgage every time it builds up. Again this will only suit if you do not need that money again.