I have a Futura 133 (2009) & I have a 9.8 sail. But to be honest I have never used, or considered using the 133 with the 9.8 sail. I weigh 75 kg and also have a 160 L formula board that I match up with my 9.8 sail.
I think you are on the correct path. I would only consider using the 9.8 with the 133 if I had most of my sailing days in super light winds with no formula board. But if only a few days a season where you can’t plane up with your 8.5 than play with tuning or technique or rig a smaller 5.5-6.0 & do light wind freestyle on those days. I just don’t see much gain pairing the 133 with a 9.8.