Is it likely there will be an Point to Point 11ac product or will it require a to complex antenna design?
Because 802.11ac is like 802.11n in terms of the basic technology, there won't be major differences in how you design point-to-point links. You will have to select a channel, and will have more options for channel width, but I think it's unlikely that many P2P links will be able to support 256-QAM.
I don't think this is likely in P2P products, which are primarily used outdoors. There are already a variety of 60-GHz. and other unlicensed P2P products offering high throughput, and as a good number of licensed systems as well. 8x8 would be too expensive and complex for broad deployment, and one would need a lot of multipath to make it work well, precluding its use in sparsely-populated areas.
Eight spatial streams requires a lot of DSP work to combine the eight streams at the transmitter and separate them at the receiver. DSPs are pretty power-hungry, so it's hard to see battery-powered eight-stream devices in our future. And it's hard to see a lot of eight-stream devices being built just for the outdoor market. As Craig says, you'll need a lot of multipath to get eight streams, and the best P2P links are line of sight and won't have a ton of multipath.