I suspect you will be disappointed by exhaust mods alone for gaining any power. I have a friend that installed the Free Flow exhaust kit from Riva on a FX HO, and it made the ski unbearably louder, but absoutely no gain in speed (we check it on a gps). Opening up the exhaust made his ski produce a sound that I would describe as a "drone" that would give a deaf man a headache in a few minutes. He sent the whole thing back.
I think you have to go with additional mods to have any chance for small gains in speed. I'd go with a stage 1 kit (if that is even available for the VX).