Feat is in pages 40, 42 and 43
A re-calc of the distance Pumpkin's beam crossed. According to this article, the shortest distance between Earth and space is about 100 km.
Distance the beam crossed = 100,000 meters.
For the timeframe, i'm gonna use the frames where Mine shouted her phrase while shooting at Budo, as well as the frames where the beam overpowered Budo and pierced the clouds. Then, i will put them in a word-to-minute calculator.
No. of words = 10.
Timeframe = 0.1 minutes = 6 seconds.
Speed of beam = 100000 / 6 = 16666.66666666667 m/s (Mach 48.5908649174, or High Hypersonic)
Blast Away! = Mach 48.5908649174 (High Hypersonic)