I think that you are saying that since you don't want to sail anything bigger than a 5.3 in the waves, you might as well make a big jump to a light wind 8.0 on flat water.
I am the perfect example of windsurfing being an individual sport with everyone making their own choices to suit their priorities and you need to do what works for you, but for the sake of, hopefully, an interestign discussion I will add to Jeff's obvious comment that it is a huge gap.
I look at it quite differently than your comment "because in waves, I don't need anything bigger than 5.3...". I love wave sailing and make that by far my first priority. To that end I keep pushing the limits of 83 kg light wind wave sailing. If you go back not that long ago, anything larger than a 5.0 on a 80 liter board was considered impossible to wave sail. Then came the Fish 90, Evo 92, Aero's and recently SOS rocket 99. Now even some "purists" are talking about the advantages of extending the days in the waves with bigger wave boards.
However on the sail side I have friends who still refuse to buy a bigger than 5.0 sail. My 5.8 and 6.3 are my most used wave sails as that is the average wind for my weight in the wave season. To my delight, the 7.0 SF is so similar to the 6.3 in feel that it is no compromise to sail in up to logo or mast high waves. The 8.0 SF works as well in the waves but due to the size and "moment of inertia" my 53 year old body had to make my sessions with this a bit shorter and I cut back a bit in the size of the waves and difficulty of the conditions. I do admit that it it is a bit of a stretch to call the 9.0 a wave sail. Jeff did build me a perfectly awesome wave sail and the whole set up with Aero 117 and 32 Vector wave fin does work perfectly, but it is a physical challenge for me in more than easy chest high conditions and there is a lot of load on the rig if I take a spill. So the 9.0 is more a transition sail for me.
My long winded point is that I find that it is now very easy to extend light wind wave sailing time, which to me beats the heck out of the alternatives. You could use a 6.3 or even 7.0 to fill in the gap and have a lot more time in the waves. As I said, this is just my personal view for interest.
Please send me an e-mail around the end of April if at that time you still want me to arrange to leave my 9.0 with Jeff for you to try in July.