We extended the bench too but only to 900mm and it was part of our quote to redesign the whole kitchen so I have no costs sorry. We also chose a cat 1 stone.
I would suggest upgrading not only the carpet but the underlay too. If you go to cat 3 carpet the underlay is included but Cat 2 you have to upgrade both. It was on our must have list after putting Cat 1 in our last house and not even being able to kneel on the floors as they were so hard (with 2 young kids was a must for us)
We were told it will cost about $1000 to upgrade both carpet (to cat 2) and underlay.
For our Newport 361 it was $1485 for cat 2 carpet and an additional $583 for cat 2 underlay.
We looked at a cat 3 one and the estimate I think was just over 3k