Support for 5x1-Portrait is finally here. Though the support is considered to be "preliminary" and was added to the 10.12 as a preview, and at the last minute - the performance and stability are solid.
A 5x1-Portrait setup is demanding, even in a CrossFire setup. It is only rare instances where a game can run 60fps at max settings and 5x1920x1080. Dropping the resolution to 5x1600x900 and/or dropping some settings provides significant improvement to frame rates.
I am glad that 5x1-P is finally here. I was pretty surprised to see how well 5x1 performed, compared to the performance drop between single screen and 3x1 Eyefinity. This doesn't mean you can max settings at full res - especially in DX11 games. Older titles will run just fine, and that tends to be the case in any multi-monitor configuration.
I know that AMD has a 5-port card in the works. The only announced but unreleased part in the 6900 series is the dual-GPU "Antilles" 6990. My hope is that the dual-GPU part is the 5-port card that has been promised.
This approach would make sense to me. A dual-GPU card is a niche product, and the Eyefinity6 card was niche product as well. Users who desire to have a 5x1 setup are the niche within the multi-monitor niche of PC gaming.
Considering that a five panel user really needs a dual-GPU configuration, I think it would make sense for AMD to combine these two features into a single SKU to serve the high-end of the Eyefinity market.
No matter what AMD does with the future 5-part SKU in the 6900 series, I am looking forward to swapping out my 6870 E6 to something in the 6900 series.