Modifying bikes, cars, boats etc., is something I have been doing all my life.
I would have to say that all stock shocks, are not pretty much junk. Maybe on the low end bikes, sure. FZ09 included....sub par for sure.
Most late model sport bikes, have suspension better than the majority of riders need.
Trust me, I have spent many dollars over the years, purchasing aftermarket suspension, for a variety of bikes. Some for street bikes, but mostly for street bikes, that I raced in various classes.
In recent years, bikes have improved wildly, and track days are readily available......a much better situation than in the past, when it was either ride like a lunatic full bore on the street, or actually go out and *road race...........*which required quality suspension fully sorted.
The purchasing of aftermarkets parts, to improve bikes has been going on for decades. Often the enthusiastic riders, buy a wrong combination of "good" parts to
make improvements, and go backwards. Typically, the ignorant, purchase high end hardware, and set it up incorrectly.
Many late model bikes with stock suspension, do just fine on sporty rides with a competent rider.....we have all seen the young lions, out there tossing there bikes in the weeds, even with all the latest best hardware money can buy.
The FJ09 shock on a well ridden FZ09, would be a ok, and most likely, not be a bad experience.