I just got back from playing an out of state tournament. Honestly, I don't have a single issue with the 99S. Serves are great, service returns are off the chart, ground strokes feel like I have a big margin of error, and net play is great. I don't see the extreme loss of tension, but I'm stringing fairly loose already. I think the racquet has more than adequate stability and plow and feels more solid at net than many racquets with an ounce of weight more on them; i.e. the C10.
Since I've been playing with it and picking up my swing speed, I have noticed that I don't get quite the sting life out of a set. But, playing all poly in this racquet, that is probably a good thing. When I used poly in my C10, I probably played too long with it.
The more I play with it, the more I like it.
Volkl C10 Volkl Tecnifibre NRG 17