I've bought an SSB filter for my FT-817. The filter is NO cheap at all, but I've found an used item for half price. Installing the filter is simple: open upper cover of FT-817, locate the two connectors on the low right side of the board and mount the filter.
The filter is enabled/disabled via main menu.
The results are very good: great improvement in RX, the selectivity is really much better. Suggestion for FT-817 owners that use mainly SSB: buy one, also on full price!