I hunted down a used power valve for a saber in Canada, not sure how it ever worked out for him.
i know CP is not going to offer a power valve on the sphynx anymore because of issues of not getting good performance with them. If you could operate it with a electric servo motor and custom cdi to operate it (Ignitech does this) then I think you could make a great running power valve motor the pressure operated ones work ok with snowmobiles because they never use the over rev part of the power band. In over rev the pressure drops in the pipe and the power valve starts to close killing the over rev.
keeping the exhaust port port timing low will build you allot of low end power and is much simpler and reliable