I recently got my MK6 GTI ECU done by APR. I'm also with NRMA. The other day I called them to confirm how they deal with modifications. Basically, the car is covered as per the agreed value - there is no need to list the modifications. Because of this I increased my agreed value up by $1000. Since i have approx 4 months left before I have to renew the insurance I was told that the difference in premium i had to pay was $5 something. As this difference was so small NRMA just increased it for free.

Hope that helps