Hi Steph:

Good choice of fin. If you like the feeling of the Venoms, you won't be disappointed with the 46 under the SL70.

As for your question re the SL80 vs SL70 - I did have both boards last year and now sometimes regret having replaced them but one can only have so many boards 8-)

Especially in summer the SL80 is a great board to have. Yes, you will definitely plane earlier with the 80/9.8 combo. By how much will depend also of your skills but my guess is that at your weight you will outperform the SL70 in anything below 14-15 knots, and beyond that depending on your skills. By the way, the SL80 also happily takes a 8.5 (I used it mainly with 8.4, and only jumped to the 70 when I had to go down to 7.6).

Cheers

c