Good question. Not sure I have a good answer.
If you are a good surfer and/or have wave height 1.5-2 feet or greater, I'd probably try the 7.0. That's because I think you'll find the K11'5" catches such waves without needing a lot of help. Ease of catching the wave is much of why longboards are are worth having!
The board is very stable in whitewater and climbs over the shore break nicely, so you don't actually need much power there - 7.0 would be enough in any wind. Most of the time, I just go with my 6.3 and it's plenty except for the lightest mush / winds (as below).
If you're not that great at catching waves, and/or have dinky mushy waves in light onshore winds (my situation most days), an 8.0 UL will help you catch more rides but will be a little in the way once on the wave. But those kind of rides don't offer too much in maneuvers anyway, the main thing is some added power to catch as many waves as you can. Key thing there is that you're just trying to have some gentle wave rides to commune with the sea and pacify your life. Really low-key stuff.
Also, for me, in the mush with light onshore winds, I find the easiest way for me to catch a ride is to get front-to-back facing the waves/wind so I can ride along studying what's coming. This also helps keep from drifting in to shore. I ride nearly luffing (just enough power to hold up the rig) until I see something I like, wherein I bear off / sheet-in and then as I catch the wave do a sail/body 180 into clew-first front-to-front riding the wave normally. For reasons I don't fully "get", clew-first on a wave face seems to give the most useful startup power. Even an 8.0 SF is a bit clunky for this maneuver (a little too slow on the flip and the wave tends to get ahead of the board). The 7.0 is quite a bit better for this (and 6.3 even better). Once I catch the wave, the sail is less relevant and smaller is better for maneuvering purposes. For my situation, this is where the sail size is a compromise in light wind & mush.
I don't know how the good guys do it (and I'm the only one I know who does this kind of sailing so I've had to teach myself), but to me that moment of catching a mushy wave in wind <10 kts is the only time I really want more power. That's when others advocate going to an SUP, and I go ride a bike or to the golf course! If I had to have only 1 sail for this board, it would be the 6.3 (for you, probably the 6.7). That would leave me catching a few less waves, but on such days it ends up being more about that peaceful communing with the lake thing, anyway.
I have ridden the K11'5" in flat water / lake mode, and that's where an 8.0 SF would help (especially if you're 10kg bigger than me). Not my kit of choice for that scenario, but for someone who needs a one-board light-moderate wind and wave solution....hey, it works.