Thank you guys for the replies. All the posts were very useful and gave some informations I didn't know. Yes, the ground speed increased immensely (over 100 kts) and information of reported severe turbulence was passed to all following ACFT along that route and FL.
However, my question is left unanswered and on a second thought, it cannot be answered at all, it seems. If the turbulence is such that the aircraft handling is difficult, pilot will request level change. Either climb or descent. Eventually, there'll be a FL for the "smooth" flight and it can be within the RVSM airspace. In that case, reporting UNABLE RVSM DUE TURBULENCE is improper and useless. In the other hand, a simple request for FL280 or 270 may solve the problem as well.
Still, it raises another question. What's the point in UNABLE RVSM DUE TURBULENCE in the first place? I doubt I'll hear that one in my career...