The limit does not exist.
"Driving my Sparrow at breakneck speed reminds me that I'm still alive."
This is a very Hunter thing to say. Not to disparage Hunters, but I don't get it (and I try very hard to "get" things, let me tell you). I don't need to be reminded that I'm still alive. I'm reminded of that every time I die.
I drive my Sparrow at breakneck speed because it reminds me of my place in the universe. It reminds me of every force that acts on me, every "law" of motion I can or can't break. I'm most interested in the ones I can't break, honestly. Considering I am myself an aberration of the laws of physics, I rather like finding the edges of things. Not that I'm content to leave those edges as they are. I just get a sort of… thrill out of finding them.
And if I have to drive my Sparrow over a waterfall of Vex milk to do that, then damn it, I will.