According to the Wikipedia, Das Paris-Geschütz only achieved 5400 ft/s.
It fired a 234 pound projectile 81 miles, achieving a maximum altitude of 26.3 miles, not exceeded until the V-2 in WW2. And it was about as accurate as the V-2 and ultimately achieved the same military outcome as the V-2.
Yes, I corrected that in a following post.
See my # 29