I just faced the same question as you - I also needed to replace a Naish 8'11", and was choosing between the Futura 111, and the Futura 104.
I chose the 111.
My situation is a bit different though, because I weigh only ~73kg.
First, I love the Naish 8'11", it is a fantastic board for fast freeride sailing in flat to moderately bumpy/wavy conditions. I mostly used it with a 6.5, and occasionally a 5.7.
My problem with the Naish 8'11, is that, for a ~100 L board, it is very early-planing, especially when compared to newer boards.
I needed a board to fit between my Formula board/10.0 set up, and my 84 L wave board /5.6 setup. The Naish/6.5 was not early planing enough.
I decided I needed a board that could handle a 7.5, so I got the Futura 111. The board is a bit bigger than I had imagined it would be, and now I am guessing that the Futura 104 can also handle a 7.5 pretty easily, and maybe I should have gotten that.
Anyway, the Futura 111 feels _much_ bigger when sailing than the Naish 8'11". It planes much earlier than the 8'11, but isn't nearly as comfortable in chop (when powered up with a 6.5) as the 8'11". So far, I have only sailed the Futura 111 with a 6.5, and for me, that is the smallest sail I would ever use on it. When powered up on a 6.5, the board is pretty fast, but a bit bouncy. My guess is that with a 6.5, the Futura 104 would be faster. I think that where the 111 will really shine, is with a 7.5 or 8.0, and I hope to try it soon.
So, at least at my size, I think the Futura 111 is a board for 6.5 to 8.0, (maybe 8.5). It really is a much bigger board than the Naish 8'11", and is not a direct replacement. I would never sail the Futura 111 with a 6.0. On this website, Starboard claim a range of 5.0-8.0, but I don't think that is very realistic. I would be _totally_ out of control in my typical 5.0 conditions, which are quite bumpy/wavy. Again, 6.5 is absolutely the smallest sail I would put on the board.
I don't want to sound negative about the 111 - i think it is a great board - it is pretty fast, quite lively, and really rewards an "active" sailing style. Considering its size, it does quite well in chop - it tends to rise up and sail on top of the chop, rather than slamming into the chop. This means that with a little attention, you can go pretty fast in the chop.
The Futura 111 like wider gybes than the Naish 8'11", which can make quite sharp turns. But, the Futura stays on the plane during the gybe much better than the Naish.