I have sailed the F133 with 8.4 RSS. I'm about 92KG and would guess I would need at least 10 knots to get going. I would be using a 48cm fin, sail downhauled correctly, with just a tiny bit of positive outhaul. I dont know what your weed fins are like, however its essential you get the correct lift from the fin at your weight. Also good light wind technique (as much mast foot pressure as possible, boom on the RSS right to top of cutout) will reduce your planing threshold by a few knots as well.
I now have the new RSS MIII's and they certainly have another .5m of grunt for each sail size. So if you can get your hands on a 8.6, that would also make a difference.
I think you can get more out of your 133, before considering another board. If you go for a larger board, you may find yourself wanting a 9.5!