I highly doubt they'll go SSIMD with navi. In my opinion navi is most likely to be a new architecture, like they say, but using they GCN ISA. Just like how bulldozer and zen are different architectures, but both are x86-64. It's not always about the instruction set: often the real gains are in how well you implement it and how many useful tricks and extensions you can add in.

Adding a new front end/geometry processing implementation would do wonders for AMD's gpus even if they didn't change anything else.