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
http://www.magix.com/index.php?id=22...ontent=vegas14

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.