I use 7.8 2009 Redline and I love every minute. It is a very light sail, had no problems whatsoever with the cambers. it is quite powerful as in the winds 15-20 knots I can go as fast as guys on 9.5.
Unfortunately I have no experience with the other sail you mentioned, but if Naish improved Redline than Grand Prix will be a great sail. Shame they are not producing them in 9.5-9.8 sizes.
Angulo Magnum 83/Angulo Magnum 60/Formula 160