The R1 '15-'16 rear shock will fit with some modifications. First, the upper eye retaining bushings must be turned a -1.2mm each to allow insertion into the frame arm. Second, because of the nitrogen reservoir, the voltage regulator tray must be cut and the voltage regulator re-positioned. The jury is currently out as to whether there is enough slack in the harness to do this efficiently.
Finally, the spring rate on the shock is 503# (or 513# on the 'M') which is good for a rider less than about 185#. While I have not yet found listings on accessory springs, the size and available preload adjustment range is similar to the '04-'08 series. I am currently in the process of adapting the shock on a GEN II FZ1 to see what is affected.
On the plus side, the '15-'16 shock has an adjustable lower clevis which allows ride height adjustment.
On the minus side, the shock design (and OEM cost) is a lot cheaper than the '04-'08 series, comparable to the price of an OEM FZ1 shock.
Currently, the eBay '15-'16 shocks are limited in number and sell at a premium. Given the issues with modifying the V/R position, the use of the earlier R1 shock with an adapter still appears to be a better and more flexible alternative at an equivalent cost..