Correct wire size for forestay?

[Kelpie]

Hi
I am looking to re-rig my 33ft S&S designed keel/centreboard yacht. She is currently rigged with 6mm all round, including the forestay. There is a profurl roller jib on the forestay and I am wondering if 6mm is not a bit light? I have been informed previously that a extra weight of a furling system requires a heavier forestay.
The boat has a design displacement of about 12,000lbs.
The jib is a bit smaller than she was designed for, as the boat has been converted to carry a self tacking blade rather than the original overlapping genoa.

Would there be any disadvantage to increasing the size of the forestay (other than cost).

Thanks