I have Vegas Platinum 12 (MSP12). This can do 4x but only as a step i.e. no ramp-up. There is also some hack which enables 12x.

I believe Pro can do a ramp (a velocity profile is I think what they call it). This is on a special offer for $200 until the end of July 2016

But it still can't go faster than 4x, though apparently one can have multiple serially connected projects to get higher values.

There is also other software but I am quite used to Vegas, and all video editors are very quirky in how they work.