Proud owner of a rear YSS here. Highly recommended.
Regarding the front: I know I'm spoiled with my several Öhlins equipped road and race bikes, but I would never do the good old "just progressive springs + thicker oil" treatment. This don't make the front "better", only stiffer. There are fork valves (emulators), could be installed with a little mechanical knowledge, and they're the best things (on the budget) what can happen to these damper rod forks. YSS have the proper sized fork valves for the FZS600. Forget progressive springs, put in linears, for your weight. Progressivity must be adjusted with oil level.
My FZS on YSS suspension front and rear behaves very good on the road, plush, but don't collapses on hard braking or uneven tarmac. I was surprised how good it is. Recommend to put a cable tie on the right fork tube to check the fork travel.
Happy tinkering!